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The Baneblade superheavy tank in Warhammer 40,000 looks absolutely sweet and its stats on paper are overwhelmingly awesome. After all, it's ready to unleash ELEVEN BARRELS OF HELL! However, the sheer ridiculous points cost means that the opponent can field a much larger force, with all the dedicated anti-tank weaponry needed to make the Baneblade into eleven mountains of scrap metal.
Some people actually pointed out that due to the rules for coming in from reserves, a Baneblade technically can't be put in reserve, despite it being an option (and the only way to ensure it doesn't get scrapped in Spearhead). This is because the literal wording of Reserves means that the unit has to move from off the board onto the board using its normal movement allowance. A Baneblade moves 6 inches a turn, it's almost a foot in length!
Then came 5th edition which not only made it cheaper (from 650 to 500 which is cheap in 3,000+ point games it's made for), but also harder to kill. It still has its limits — sinking 500 points into a unit which couldn't capture objectives at the time, is relatively slow, and is vulnerable to rear armour hits like all tanks — but it's at least usable. Not to mention there's nothing better than the look on a Guard player's face if you do succeed in wrecking his precious baby.
A lot of special characters fall into this, especially the ones that are pure beatsticks. They are extremely powerful, but also extremely expensive and, if not immune to instant death, die to a single tank round. That's why you don't see the likes of Typhus or Marneus Calgar very often. The most used special characters either buff your other units (like Vulcan, Ghazghkull or Fatereaver) or are GameBreakers (like Eldrad).
Specifically, such awesome but impractical characters are called Deathstar Units but not for the reason you'd think; They are massively powerful, incredibly huge targets, ridiculously expensive, require a large tax on all of your resources...and have a laughably easy weak spot for all that it took to get them on the table.
The Baneblade (and its brother Super Heavies, the Mjollnir and Excalibur tanks) didn't fare much better in the larger scale game Space Marine. True, their 1+ save meant they could only be taken out by reasonably powerful weapons, and they packed Titan class weapons... but for the price of one of them, you could buy one or two units of regulars tanks, which could 1) bring more guns to the party, 2) move much, much faster and 3) not die in a single lucky hit. For that matter, the Ork Super Gargants were also all but useless - being extra hardy is nice. Having only short range and contact weapons on a slower-than-tectonics bullet magnet ? Not so much.
You could say 40k was BUILT on this trope. The basic heavy weapon for the Imperium is a hand held semi-automatic armor piercing grenade launcher, chainsaw swords abound, the Orks function purely on Clap Your Hands If You Believe and duct tape, the Eldar have slightly better protection than a cardboard box while retaining maximum style points, and the space ships get around by travelling through hell. It's all awesome, but none of it's remotely practical.
Actually, Eldar tanks are actually as durable as their Imperial equivalents. Eldar lack heavy tanks like Leman Russes or Land Raiders, but compared to Chimaera-and Rhino-chassis tanks (which, like the Eldar tanks, are based on a transport chassis), Eldar tanks are actually more heavily armoured, faster, and have additional defences in the form of power- or holofields. They're still impractical, though, due to their extremely high cost (a Wave Serpent costs nearly four times as much as a Chimera, while having similar firepower and being only slightly more durable).
The Grey Knights is this trope embodied. Not only is the average Grey Knight Trooper more expensive than its loyalist OR Chaos Counterpart (in both in-game point cost and real-life money costs), it still dies by the same weapons that can kill the aforementioned cheaper counterparts. The Awesome part? Even the basic trooper is armed with an appropriately named Nemesis Force Weapon and Stormbolter, which turns each and every Grey Knight into a Lightning Bruiser / Glass Cannon that can tear apart a tank in close combat....if you ever got it that close to a tank. It's even lampshaded in their codex, which mentions that it would be very hard to play a pure Grey Knight army (although possible).
This is made even more glaring as most of their stuff is now overpriced and out of date by current marines who, while still being much cheaper than a Grey Knight, has much more equipment to compensate for any of their earlier lackings. Chaos Space Marines also got an update so that they can be upgraded to be harder to kill.
This is just not the case anymore. The 2011 Grey Knight codex moves the army into Game Breaker and Tier-Induced Scrappy status. 55 point two wound terminators with force weapons that have 4+ invulnerable saves? That claim objectives?
Feel No Pain on the paladins (the aforementioned 2 wound terminators) is itself an example of this trope. Terminators are already very well protected from low strength and AP weapons, paladins more so as they have two wounds and can abuse the wound allocation rules. Therefore, most fire directed at them will either be AP2 or S8 (or both), negating FNP. Considering how much a paladin apothecary costs, it's hardly practical to get them FNP instead of just buying more paladins.
The Deathstrike Missile. It's ungodly powerful (being one of the most powerful weapons you can field in an non-apocalypse game), indestructible, and has a huge blast radius. The impractical part? Due to being a blast weapon, it's inaccurate to the point of suicide (unless you fire it across the table, it's fully capable of flying backwards and obliterating your own troops). Doesn't help that it's being crewed by normal guardsmen, who are not known for their accuracy. You also only get one shot per game. It also can't fire first turn (leaving it open to being shot off the table; It's not that well armored) and is only really useful against less mobile, more concentrated armies (playing Dark Eldar? You'll probably hit 1 unit with it).
Speaking of Apocalypse, both of the Lethal Joke Characters. The Warlord Titan is three times the size of a Warhound Titan, which is already gigantic even in miniature scale. There is also rules for a Imperator Titan. To say it's easier to cosplay as one and just stand on the table instead of making a model is NOT an understatement of how big it would be. In fact, it may actually be smaller than making a true-to-scale model. You would need reinforced tables just to use those things.
Imperator is a sub-class of Emperor Titan, along with the Warmonger Titan. They are both roughly the same size. Warlord Titans are one step smaller, followed by Reaver Titans and finally Warhounds. Games Workshop have only ever released Warhound Titans (and, from Forge World, Reaver Titans) in 40K scale. The Imperator/Emperor model was only ever made in Epic scale, along with all the other Titan types.
The 40K scale Tau Manta transport gunship from Forgeworld. At a massive 63x86cm (25x34 inches), this thing not only does not belong on a table, it is a table. And at nearly a thousand UK Pounds, there aren't likely to be too many players who can afford one.
On paper the Fire Prism looks pretty awesome. It is tougher than a Land Raider when fully upgraded, can fling S9 laser bolts around, and can combine fire with other Fire Prisms to make even bigger superlaser-of-death shots. Then again, in smaller games a single 90pt War Walker can hit tanks harder than 2 115-pt Fire Prisms combining their shots, a Dark Reaper squadron can kill heavy infantry nearly as effectively, and a Falcon can blow more things up for the same Heavy Support slot while remaining just as tough. Thank Khaine for Carnifexes, because if there were no Monstrous Creatures the Fire Prism wouldn't exist.
In the 6th edition Eldar codex, this is no longer the case, as the Fire Prism has moved to Awesome Yet Practical. It's gun has 3 firing modes, a S5 AP 3 Large Blast, perfect for blasting apart large units of everything from Marines on down, a S7 AP 2 Blast, perfect for Terminators and the like and an S9 AP 1 Lance, perfect for killing tanks. The clincher? All 3 of these have a 60 inch range and can be fired after moving 12 inches, meaning that in a board of any size, it's very difficult to return fire on one. Awesome Yet Game Breaking might be a better term.
In a similar vein, the Land Raider itself is this. On paper the thing looks damn impressive, having enough firepower to destroy any other tank in the game, or take on a full squad of elite troopers, while its armor laughs off swarmers and anything not dedicated to killing it. Its point cost, while high, is still affordable in most standard games. The problem is, the primary perk of a land raider is to survive, which does little for something that cannot 1.) do that much damage and 2.) capture points. For the same cost as a land raider, you could have afforded two tanks with twice the amount of weapons, and are viewed as less of a target.
Many a vanilla unit is prone to find itself in this role. Consider assault terminators. A Codex Marines army can spend 400 points just fluffing out a ten man squad. Sure, they'll hit like a ton of bricks if they catch anything in melee, but because of their high value and slow movement, they aren't going to catch many things in melee at all, and certainly not enough to make back their points. The Marine player will likely find that unit has a hard time getting in close with anything that matters, meaning the enemy can avoid it if they want, feed it a low-value unit if they must, and pick apart the rest of the Space Marine army with their force. A smaller terminator force would be far more dangerous because it will be supported by other units who can keep the enemies from having a whole table to maneuver on. There's nothing sadder than finding a new Space Marine player put his two hundred point HQ into his 400 point assault terminator squad and all they did was take down a squad of Red Shirt Imperial Guardsmen.
Similarly, Imperial Ironclads. Ancient warships from before the development of Deflector Shields, they protect their occupants with tonnes of armor. Because they are built in the times before or during the Dark Age of Technology, they often have apocalyptically powerful armament, but their lack of shields makes them Glass Cannon units at best. The Imperium usually recognizes this and converts them into either planetary assault ships (fit some rudimentary deflectors, fill it with guardsmen and bombardment weapons) because they can actually land on planets due to their freakishly strong construction, turning them into Boring Yet Practical. Other uses include stripping out the weapons, replacing them with more bolstering, adding shields at the front, massive engines at the back and using them to ram things, or replacing everything with a BFG and using it to kill planets. Some of the very youngest ironcladsnote Or those the Imperium can viably fit proper shields to. fall into Awesome Yet Practical - they keep their Dark Age of Technology-level weapons, but their plate armor is good enough to stop "modern" weapons, and glows red hot with each hit, making them lookawesome as they advance into battle.
Assault Centurions have some of the beefiest armour in the Space Marine list, cool-looking siege drills, and a movement speed roughly akin to molasses. This is a melee unit that can barely outrun the scenery. You could always put them in a Land Raider, which has the minor problem of being akin to walking into battle with a "SHOOT ME" sign.
In-universe we have the Penitent Engines, machines bristling with devastating close-range weaponry - and whose pilot is intentionally left unarmored and fully exposed to enemy fire as a form of penance for past crimes. The enormous waste of resources of building a mech that could be put out of action with a well-placed headshot is apparently unnoticed in the WH40k universe.
In Exalted, the Sidereal Exalted have the reputation of unparalleled Martial Artists. They can create and learn Kung Fu styles so powerful that they rewrite the reality at whim and so flashy that fans of all the other splats demand them to be universally available. The catch is, these Martial Arts are incredibly expensive and very cumbersome to successfully employ in combat. Most Exalts are much better off using their less awesome but cheaper and more reliable Kung Fu.
Not to mention, the Five Metal Shrike. It's a flying machine which survived the First Age. It's smaller than any major weapon of the First Age, and more maneuverable than any flying machine in the Age of Sorrows. It's got Artificial Intelligence. There is only one. It can repair itself, it can respire Essence by itself, it can cross Creation in a day, and it can become completely invulnerable to damage. In addition, its main weapon is the "Godspear". Listed statistics include: "infinite levels of lethal damage". Unfortunately, only one exists, it's controlled by a machine, and it can only fire the Godspear once per day. Oh, and creating a new one takes over 100 years, assuming you're already got the required knowledge. And the infinite damage radius is only 25yds, with a secondary blast of "only" 50 damage in a 500yds radius (where most characters have health levels in the single digits). Oh yeah... if the Shrike is destroyed, it explodes with double damage and more than triple the radius of the secondary blast from the Godspear.
The Titan-class Citadels the Shrike was intended to replace are the Shrike, needing a crew of 5,000, with a primary weapon one mile across, with an explosion that flattens a truly ridiculous area when destroyed. In other words, the Shrike is practical by comparison, unless you actually want to train up five thousand random villagers.
Any combo attack against a creature that has a perfect defense.
And then there's sorcery, which teaches us that control over the fundamental forces of reality, including demon summoning, rains of apocalyptic death, and showers of obsidian insects to tear everything to shreds...is, ultimately, overshadowed by the native powers of the average Exalt. High-level Terrestrial charms (as in, the weakest of the Exalted) such as "Dragon Vortex Attack" and "As In The Beginning" are substantially nastier than even the spell Total Annihilation. When the right kung fu combo on top of a mountain can destroy the world, sorcery just ain't that practical.
With the 2.5 errata, it turned out that the Infernal charm trees were neat, interesting, cool character-building for the Yozis, and completely unsustainable inside a semi-functional version of the Exalted ruleset.
Subverted by the Gloryborn template armour from the Dungeon Master's Guide II. This was explicitly stated to look as absurdly impractical as possible - men would get chest armour limited to leather straps to show off their pecs, while women got what you'd expect. Despite looking Awesome But Impractical, it really functions exactly like normal armour of that kind, due to the extra-planar origin of the equipment.
Also in this realm are many tricks the character op boards can come up with, for example a sack of pieces of paper with explosive runes cast on them, then cast a dispel effect on them and purposefully fail. In theory it does massive damage, in practice, the cost and time involved make it a lot easier to just exploit the game's inversion of Useless Useful Spell and fling save or dies around.
Also the Thrallherd prestige class gets legions of mind slaves, replenished each day, using them as suicide bombers is a neat gimmick, it isn't that great mechanically.
The Wish spell, at least in 3rd Edition. Finding wishes in premade rings and other devices was useful but given the limits placed on it, the potential for backfire, and the 5,000 XP cost, it was almost never worth it to cast it.
Wish in 3.X is a Panic Button, nothing more. You only use it when the party is about to die, and only to duplicate a spell effect (one of the few uses that doesn't give the DM much room to put a drawback on). And the XP cost? You get a third of that every encounter at 17th+ level play, so it really doesn't hurt that much (plus any optimizer will tell you that losing a full level isn't always a bad thing as it lets you gain more XP than the rest of the party).
The Dungeon Master's Guide explicitly instructs dungeon masters to pervert even the most benign of wishes (it claims not to, but then it gives examples where it does). Many take it to heart, making the spell an effective suicide method.
The Deck of Many Things is an artifact that has an equal and random chance of completely destroying the character, or instantly granting it great powers (level up 10 times). The latter is less useful than it sounds, since you will be too powerful to play with the rest of the party, you would thus have to retire that character regardless of what you draw.
The Epic Spell rules allowed amazing customization for spell creation, and the example spells were all over the place. Examples include Nailed to the Sky, which teleports the target into orbit; Dragon Strike, which summons ten red dragons, but requires eleven other casters, minimum level 17, to burn 2,000 XP each; and Vengeful Gaze of God, which deals massive damage to one target and blows 2/3 of it back in your face, unavoidably.
Vengeful Gaze of God causes, on average, 1067 HP of damage, to any one target within a 4 kilometer radius. Pretty big, no? Except... casting it requires a skill roll with a Difficulty Class of 419 (probably the highest DC ever to show up in any D&D book), which is impossible even to a wizard on level 100. And if you are powerful enough to cast something like this, then at your level even the weakest mooks you face probably have hit points going into the thousands. And for added fun, it causes an average 700 HP of backlash damage to you.
Third Edition had the "Brilliant Energy" weapon ability. At a staggering +4 price mod (a weapon's total cost was it's price mod, squared, multiplied by 2000 gold pieces), a Brilliant Energy Weapon ignores all inorganic matter when swung, sailing effortlessly through armor. Sounds cool, right? The trouble is that most of the enemies a player will face by the time he can afford it see it as inconsequential; Outsiders have massive Deflection-based defenses that don't use matter, Dragons have incredibly thick hides that a Brilliant Energy weapon won't help pierce, worst of all, undead and constructs don't have any organic matter at all—meaning that the weapon will sail through their entire bodies, leaving them completely unharmed.
On the other hand, that last bit means a Necromancer capable of casting the spell that gives people Brilliant Energy weapons (temporarily) has an unstoppable army.
This is the calling card of the Tier 2 classes: the sorcerer, the favored soul, the psion, and so on. Each has access to Story Breaker Powers, but they simply don't have the wealth of options or freedom of choice that a Tier 1 has: either they start with a comparatively smaller list, or they have a smaller number of powers that they can actually use (a wizard can prepare Water Breathing or Knock easily, but a sorcerer can't afford to waste his spells known). They can still easily dominate, but they often need to stick to Boring but Practical choices or risk Crippling Overspecialization.
Savage Species is infamous for this. It's a book that turns monsters into classes that players can start as, meaning you can end up starting out as things like mind flayers or chain devils. The problem? You can't advance into anything else until you've finished the class, for obvious reasons, you can't take another race, and thanks to Level Adjustment, your HD, save bonuses, and feats are all lower than a normal character of your level. This last one meant that a CR 5 mummy was now treated as equal to a 13th-level character. As if that wasn't enough, the monster classes were all this trope. You could classify them pretty easily into the following categories:
Monsters with such a powerful chassis and enough good spell-like abilities (usually outsiders) that they can easily hold their own, or even outdo other classes for a few levels... then the HD limits kick in, and they fall behind.
Monsters geared for brute melee combat (ogres, trolls, etc), which usually end up as passable melee workhorses markedly inferior to a same-level human fighter or barbarian.
Monsters which rely on a gimmick attack (medusa gaze, undead spawning), which don't actually get their signature power until a long time after it stops being dangerous (mind flayers get Extract at 12th level, a long time after clerics got Slay Living) and spend all other points in their career making subpar melee attacks.
Although it functions just fine in-universe the dire flail weapon would be this in real life. It is basically a wooden pole with a chain and spiked ball on BOTH ends. How on earth someone is supposed to operate such a weapon (at least in such a way that it would be superior to a single-ended flail) is a mystery since swinging one end at the opponent would almost certainly imperil the wielder with the other.
A Fourth Edition example: about a third of the various Paragon Path choices, and easily half of the Epic Destiny choices. Oh yeah, you might be able to do things like make three rolls to bluff a city into thinking you're an incarnated god, or be able to walk back home from the afterlife... But there's virtually no practical combat help (with powers roughly equivalent to ones you'd pick up anyway) and a too-broken character is just a big target for the DM to figure out what not to have happen in the story lines.
In the board game Risk, the player who controls the whole of Asia gets a bonus of 7 battalions per turn, the largest bonus in the game. This might be useful save for the fact that you almost certainly don't need the bonus if you are capable of successfully holding Asia for a turn (with its many border provinces).
In the Buffy roleplaying game, they mention that chainsaws are realistically very impractical weapon, but because so many movies show them off to great effect, they have it deal a lot of damage, but they take penalties to hit and they can hit the wielder if you botch.
In the New World of DarknesssourcebookArmory, chainsaws have a penalty to use and hurt you if you botch, but if you do hit, you get Eight Again on the damage roll (in other words, you reroll successes until you get a roll without any successes, then add them up - every success is a level of damage).
The monofilament whip in Shadowrun is a cyberpunk vorpal sword: A filament made of a single long chain of molecules with a handle at one end and a little weighted ball at the other. Presumably it is so sharp that it will instantly sever a limb, but you need mad skills to use it; if you miss your target, the whip is likely to come back and take off your head.
A Rifts example is the Cosmo Knight from the Three Galaxies sourcebooks. At first glance, they look like the cutting edge in Munchkinism. They take 1/100th damage from energy weapons (meaning that you need a starship-mounted cannon to even ding their armor), and that's on top of the massive amount of damage they could soak up even if they didn't have that particular ability. That's on top of a stack of other wild abilities they have, and their "one great" weakness is a total joke (they take full damage from magic). The drawback? You have to play the character as Principled alignment (Palladium's equivalent to Lawful Good), and on top of that, they have a strict Code of Honor they have to follow. Unless the GM is lenient (and how often does that happen?), step one toe out of line and the character becomes a Fallen Knight. Fallen Knights lose most of their powers, and while still tough, aren't a patch on what they used to be. Many a Hack and Slash power gamer has thrown the Cosmo Knight away in disgust upon learning that they couldn't just rampage through the galaxy, killing anyone who so much as looked at them funny.
Lots of things simply aren't legally available at your security clearance, forcing you to risk the Infrared market to get them at all. Or the thing itself is available, but the instructions on how to use it aren't.
A prime example is canned food, which is normally a red (second-lowest) clearance item. All Troubleshooters are at least Red clearance. Can openers, however, are at least Blue clearance (four levels higher).
Tactical nuclear grenades, with the helpful instructions "Throw Really Hard" printed on the side. Maximum throwing range of a typical Troubleshooter: 50 meters. Blast radius: 500 meters. Alas, no Troubleshooter has survived long enough to report this particular design flaw...
Flamethrowers and (especially) plasma generators do lots of area-of-effect damage, but when it malfunctions (which it does relatively often) or a shot hits the fuel tank, it tends to explode - and it's strapped to your back, so you can't run away unless you take time to unstrap it first.
Experimental equipment from Research and Design ranges from relatively useful to downright harmful, with this trope somewhere in the middle ground. Troubleshooters are often tasked with field-testing some R&D equipment while carrying out their primary mission.
Warhammer Fantasy gives us the Steam Tank. Looks cool, is utterly worthless. If it takes any damage, its rules render it unusuable.
The damage will mean you either have to play more conservatively or risk catastrophe, but even if the opponent manages to sneak in a couple of wounds (almost always needing to roll a 6 to hurt it, and then you not rolling a 3 or more to discount the wound) it will still chew through enemies in combat. As long as it picks its fights well (ie: trying to avoid hordes who hit hard or being sniped by enemy cannon, instead going for small blocks of monsters or elites) then it can be a holy terror.
Every named character who isn't a squad upgrade. You can build a normal character to outdo any named character regardless of who he is, and some of the named characters are so absurdly expensive that you can put together two tricked-out Lords for the same points cost (Malekith, we're looking at you).
The very act of charging became this in the latest edition. Previous to this, charging at least guaranteed you the ability to attack before you died. Now, without special rules, it nets you a measly +1 to combat resolution, i.e. nowhere near enough to cope with the casualties you just took against that high-Initiative opponent.
"Storm of Magic": Exalted Greater Daemons and Emperor Dragons. Exalted Greater Daemons are powerful, very resilient, usually spellcasting monsters capable of eating entire units, but they cost so much that even a 3000pt army cannot bring even one with its Monsters and Magic allowance. Emperor Dragons, at least, are cheaper in that they only cost 650pts, so a 3000pt army can bring one, and they've got a whole bunch of stats at 9, but they're still more than a 2000-2500pt army can afford. Both, in addition to this, are vulnerable to not just concentrated war machine fire, but also the "Unbind Monster" cantrip, which offers a 1 in 6 chance of killing a bound monster outright from 24" away with no saves and can be cast on a 3+ by any wizard on the battlefield.
The Cataclysm spells in Storm of Magic are all weakened by their inability to get Irresistible Force, meaning that they can always be dispelled if your enemy has enough dice, but the worst has to be the Lizardmen's Great Leveller. This spell forces the enemy to sacrifice units and characters until both armies have the same number of each, but it has a laundry list of drawbacks - it automatically kills the Slann casting it, it can only be used if you control the majority of Arcane Fulcrums, which is unlikely if your enemy is in a dominant enough position that you'd want to cast it, and worst of all, its 35+ casting value was set while Slann had a game-breakingly powerful way to cheat the cap on dice rolled per spell, which they have since lost, meaning that trying to pull it off under current rules requires a roll of at least 28 (assuming favourable Winds of Magic) on six dice (average roll 21).
Starfleet Battles had battleships. Huge and powerful, and enough to make your oponent look for his brown pants, they were horrificly expensive (especially in campaign play), and effectively required an expensive support fleet to protect and augment it. In the end, 'historically' only the Klingons ever built any, and only two of a planned nine.
Battleships also suffer from turning like a drunk pig. Even the Klingons, renowned for building some of the most maneuverable ships in the game, couldn't do much about that problem, and their battleships are among the more maneuverable (and turn as badly as anything else classed as 'real' in the game). When Federation (conjectural) battleships were introduced, a new (worst) turning mode was introduced for them. The problem is so severe that all battleship designs include rear-firing heavy weapons to discourage getting behind them.
To a lesser extent, the Stasis Field Generator and the Mauler Cannon were powerful, but only useful in certain circumstances, and required a skilled player to get any effective use out of them.
In Warhammer Fantasy Roleplay, half the highest-level careers are this. The Wizard Lord, the highest-level magic user, requires the character to possess 6,000 gold crowns' worth of arcane tomes, in a game system where the players are playing wandering sellswords and obtaining a single gold crown is a grand achievement.
The Rube Goldberg device at the center of the game Mousetrap. It is fun to watch in action, but the "trap" takes a long time to assemble and NEVER works from start to finish. Fortunately, nothing practical is provided.
GURPS tends to be full of this. Owing to the game's tendency to strive for "realism" even when it is emulating unrealistic genres, many "cinematic" maneuvers may be allowed but are still modeled as being almost as impractical or dangerous as in real life.
BattleTech had mechs such as the Clan-tech Blackhawk (appropriately called the Nova by the Clans) which packed 12 ER Medium Lasers. If it gets in range, it will take down virtually any mech its size and smaller, and possibly many larger. But a full salvo generates so much heat the mech risks temporary shutdown from the first volley. The reason for this is because it's a dueling mech: it's built to fight one on one with something and chip away until it gets an opportunity to make a spectacular all-or-nothing Alpha Strike.
Many weapons fall under the Awesome but Impractical category as well. Melee weapons, for example- a hatchet does great damage for most mechs, but to use it you've got to be adjacent to your opponent, and they know you're packing a hatchet. All too often the amount of tonnage devoted to packing one just ends up as dead weight that could have been used for longer ranged lasers or missiles. Another good example is the Hypervelocity Autocannon: better range than standard autocannons of the same damage potential, but they're also heavier (a real problem in a game in which weight is the main weakness of ballistic weapons relative to energy- and missile-based ones already), generate more heat, and have a 1-in-36 chance of blowing up each time they're fired.
The old West End Games Star Wars RPG played this straight and averted it for different characters when it came to light sabers. If you had the skill (which only a Force-sensitive character with access to Jedi or Sith training could acquire) then a light saber was an AWESOME weapon, generally quite capable of shredding anything that operated on character scales, and pretty dangerous for vehicle and starfighter scale opponents as well. If you DIDN'T have the skill and tried to use it, a feature of the rules meant that there was a 1 in 6 chance each time you made an attack that something really bad would happen, such as lopping off one of your own limbs. Or you head. Or a friend's limb or head.
Wizards of the Coast's own version of a Star Wars RPG (Saga Edition) came out with one particular textbook case of this trope. In the "Force Unleashed" supplement, they wheeled out the rules for Unleashed abilities, which essentially allowed you to pull of Superhuman stunts on par with those Starkiller used in the game, and they even came in varieties for non-force users. The problem was twofold: Firstly, you had to use up one of your feats to be able to use them at all, and secondly, you would almost never use them. Each usage of an Unleashed ability cost the player a Destiny point, of which you only ever gained 1 per level. This meant that you were wasting a valuable slot on an ability you might use a handful of times throughout an entire campaign.
Maka Dai Dai Shoji is a Japanese chess game that dates back to the 15th century. It has 361 squares, 50 different types of pieces whith 96 pieces on each side. There are at least three types of King (such as King, Dragon King, and Free King) along with an Emperor and Prince. A game like this does not last only one evening. There are many pieces which represent cool things like the phoenix, dark spirit, evil wolf, she devil, guardian of the gods, and coiled serpent. Some pieces like Emperor are all powerful (they can instantly jump to any unprotected square or enemy piece on the board). Many of these pieces may never get to be used in the course of most normal games played by experienced players. Needless to say, the more compact versions of shoji were always more popular in Japan.
The HMS Dreadnought from the Pirates Constructible Strategy Game; it possesses five masts and five cannons that hit on a 4+ die roll, and these cannot be eliminated until all of its masts have been shot away. The drawbacks? 26 point cost (when most games run 40 point fleets) and slower than a glacier when sent out with no crew. Giving it a captain and a helmsman will tack on 5 more points, bringing the total cost to 31 points. While it might have a chance of doing some damage, if its sunk, captured or wrecked, then half the player's fleet is gone.
The USS Constitution has a similar problem, although it is more affordable than the Dreadnought at 22 points. It's also slightly faster, but still very expensive to properly man.
Blood Bowl ogre and vampire teams look like they would be broken at a glance. Players of both types are outright Lightening Bruisers, but rules regarding the former's stupidity and that the latter's bloodlust mean teams composed them are doomed to fail.
In AT43, the UNA subgroup M.Ind loves combat striders, the heavier the better. The UNA army book mentions that some over-eager M.Ind generals actually deploy units with two support striders (the largest type available). This is noted to be extremely expensive and more than a little ridiculous, both in-universe and in actual play. Among other things, support striders present large targets and are intended to support other, smaller/faster striders.
The device Tesla installs into the torch of the Statue of Liberty in Rocket Age, during Trail of the Scorpion. The upside: It can wirelessly power the entire eastern seaboard. The downside: If it overloads it would wipe out New York.