With hundreds and hundreds of cards, The YuGiOh card game has so many cool cards that just aren't really worth the effort that it gets its own page.
Keep in mind, in the right deck, most of these examples can become Awesome yet Practical.
There is a series of WATER monsters that are unaffected by Spells as long as Umi is on the field. This is great against cards like Raigeki and Dark Hole, until you realize that they can't be targeted by any Spell Cards, including your own.
Most Tribute Monsters (monsters with 5 or more level stars) in the card game. You can spend an enormous amount of resources on it and lose the whole thing to a simple Bottomless Trap Hole.
A good deal of this problem had to do with most skilled players having, at almost any given time, at least three or four different ways of destroying, banishing, returning to the opponent's hand, or otherwise neutralizing any such monster. Anything that requires more than one tribute to get it on the field just isn't worth the effort considering how likely it is that your monster will be a victim to this.
Those monsters made more sense earlier in the game/anime, where attack points counted for more, as it stands now even signature cards like the Blue-Eyes White Dragon have become anachronistic and require some kind of defense to help them survive.
And then there's the anime-only card, Ragnarok. The effect? If Dark Magician,Dark Magician Girl,Dark Sage, or Magician of Black Chaos (a minimum of 2 required) are on the field, all monsters on the enemy's side of the field can be banished. The cost? You have to banish every monster from your hand, deck, and graveyard. The cost was probably only there for the undoubtedly awesome visual effect: all of Yugi's monsters appear and swarm the enemy in order to banish it.
Well, as impractical as this sounded, it was able to defeat The Winged Dragon of Ra, bring Dark Marik to his knees and rescue the real Marik. Not exactly easy. Desperate times call for desperate measures...
Also, remember that the card doesn't say you can't use those Spellcasters to attack after (in fact, everything on your field is untouched, it just cuts your reinforcements), essentially meaning that if you're in a game running on the 4000 Point system, you can easily wreck up your opponent's Life Points and end the entire game.
There's actually a real-world version now. It has the same cost but any two Spellcasters work. The downside? It only destroys one monster on your opponent's field. If this card could ever be used successfully in a tournament, it would be a ridiculous Crowning Moment of Awesome.
In the card game, we have Final Destiny, which destroys all cards on the field at the cost of 5 discards. Since the maximum hand size is 6(baring the use of spells like Hieroglyph Lithograph and Infinite Cards), playing Final Destiny leaves you with likely no hand and no field.
This may not be as impractical as it first appears. Many Fabled monsters trigger when discarded (even if they're discarded as a cost), meaning you might be able to get out a decent synchro or xyz, and Infernity monsters require an empty hand to activate their effects. In addition, there are other cards that need to be in the graveyard to work, which can be set up like this. It's still impractical, but not as much so as it first appears.
Super Vehicroid Stealth Union, a Combining Mecha Fusion Monster made of 4 specific monsters, with an effect that lets it attack all the opponent's monsters while negating their effects... and halving its own ATK, which isn't too high to begin with.
Stealth Union doesn't win by killing your opponent's monsters. It wins by the often overlooked stealing effect, which will take down anything non-Machine while working as a crazy strong wall. Sure, 4 specific monsters is a pain in the ass, but the real problem is on the Vehicroid Archetype itself, not Stealth Union.
Additionally, Stealth Union is actually fairly easy to summon by exploiting a loophole. You can use Chain Material, which allows you to "cheat" at Fusion Summoning during the turn by fusing materials from your deck instead of your hand, at the cost of preventing you from attacking for the turn and destroying your fusion at the end of the turn. By using this with Vehicroid Connection Zone, which lets you fuse for a Vehicroid and makes the summoned Vehicroid immune to destruction by special effects(including something like the aforementioned Chain Material), you can summon Stealth Union with only 2 cards.
The banned Victory Dragon, an extremely hard-to-summon monster with poor stats that, if it somehow attacks directly for the win, wins the entire match. Not just this game, but an entire set of "best of three" duels. However, there's no rule saying your opponent can't just forfeit the duel when you attack, sparing him the match loss.
Except in Japan, where there is. That's the reason it was banned in the first place. It's still not much of a strategy, though; Victory Dragon is, as mentioned, hard to summon and lacking in the stats department, and takes time to set up: if you lose to one, it's not going to be a sudden turnaround, since Victory Dragon needs three Dragon tributes and a clear field to work. Generally speaking, if you've gotten your opponent to less than 2000 LP and you've got three monsters on the field while they have no defenses whatsoever, you were probably going to win the next game anyway.
It also helps that it counts your opponents turns towards the twenty turn count effectively giving your opponent ten turns to win instead of twenty.
Perfectly Ultimate Great Moth embodies this trope: he's the strongest Insect in the game, but his Summoning conditions are practically impossible. But a reading of the card means that the listed summoning conditions only apply to special summons; you can tribute-summon normally.
Except if you tried to pull that in a tournament the judges would just laugh at you. That's just the way Yu-Gi-Oh words cards. Plus, as of its reprint in the Dark Beginnings 2 set, an errata (clarification of the text so that people know how the card's meant to work properly) was made to clarify that it cannot be Normal Summoned or Set.
In some video games, actually managing to summon this thing unlocks something good. (In fact, you have to do something hard to unlock a lot of stuff, like summoning Gate Guardian, winning via Destiny Board, and the like.)
And now the original Egyptian God Cards are being printed for legal tournament usage. They're somewhere between this trope and Game Breaker as they can be either useless or nearly unstoppable forces of destruction in the game as they require THREE tributes to summon. Being one of the few things that requires three tributes to summon, it would make them useless if they didn't block any and all effects from going off when they were summoned. Sadly they still fall to a Mirror Force just as easily as any other monster when they attack.
Obelisk the Tormentor subverts this, being almost unstoppable with the right set-up and just really needing like 2 or 3 turns to finish the job anyway.
Winged Dragon of Ra is this trope. First, he can't be Special Summoned at all. Second, he has no protection effect, except when he's Summoned. Third, his ATK and DEF is always 0, unless you pay all your Life Points except 100 to have it gain ATK and DEF equal to what you paid. If you attack your opponent directly, maybe you can finish him off before he can do his Cherry Tapping. Fourth, his last effect allows you to pay 1000 Life Points to destroy any monster. That's really useful, but you need to skip using his ATK gaining effect to use this, which also means keeping a 0 ATK monster completely vulnerable at your side of the field. Quite stark contrast to the anime, where, ignoring the borderline Deus ex Machina it exhibited later in the arc, Ra's ATK and DEF were equal to the total of the three monsters sacrificed to summon it. Giving up life points to increase it was also an option for when you ended up with it on the field with no stats, however the user could give it however many points they wanted.
The legally reprinted Slifer the Sky Dragon partially subverts this. Its ATK and DEF are determined by the number of cards on your hand (most of the time you will not keep your hand size large) and can be run over by attacking monsters. Also, unlike Obelisk the Tormentor, it CAN be targeted by card effects. However, it can act as the powered-up version of King Tiger Wanghu so your opponent cannot summon powerful monsters as long as you have a good hand size. Also, unlike Winged Dragon of Ra, it CAN be Special Summoned even if it only lasts one turn.
Horakhty, the ultimate incarnation of the God cards, requires you to tribute one each of the originally printed God cards (again, the Gods are already difficult to summon). Although it grants you victory of the duel, most players prefer finishing his/her opponent off by just using one of the God cards to deplete his/her opponent's Life Points. Much simpler, isn't it?
There is actually another way. Only Winged Dragon of Ra says "Cannot be special summoned." So just banish Obelisk the Tormentor and Slifer the Sky Dragon, and use Return from the Different Dimension to get those out. What do you do with the other three monsters that you get back with Return from the Different Dimension? Sacrifice them to summon Winged Dragon of Ra. Better, use Gadget cards that constantly cycle themselves so you'll always have a supply.
Armityle the Chaos Phantom. It requires banishing three cards (the Sacred Beasts, who can also fall under this trope) , however since each of those three cards themselves require three cards to play, it really takes a total of 12 cards to get out. It gains a whopping 10,000 attack during your turn, meaning any successful attack would almost certainly win you the game (unless your opponent had a +2000 attack monster or really boosted his life points) since you start with 8000 life points. As if being ridiculously hard to get out wasn't enough, it only gains 10,000 attack during your turn, can be destroyed by any common methods (except battle), and is actually inferior than the combine might of the cards it requires (the Sacred Beasts combined may have as little as 8,000 attack, but usually will have +12,000 attack).
Phantom of Chaos and Elemental Hero Prisma make it much easier, because Armityle the Chaos Phantom simply looks at the name, not the original name, so unlike with Horakhty, you can fake it.
Exodia: It gives you an instant win, but only if you have 5 certain cards in your hand at the same time (The cards can also be played as weak monsters). And you can only have one of each in your deck. The only way to use him efficiently is to have a deck completely built around getting him in your hand, which is still a heavily luck-based strategy which leaves you almost defenseless especially if your opponent uses cards that discard from your hand.
Unfortunately, as of June 2013, Exodia decks have become full-on Game Breakers. Using cards like Royal Magical Library with Magical Citadel of Endymion, along with cards like Upstart Goblin and One Day of Peace (both downside of each card aren't downsides when you realize that you don't need to deplete the enemy's Life Points to win), it is common for Exodia decks to win in one turn. And if they get two turns, like when they go first, their victory is pretty much guaranteed.
Similarly, Exodia Necross. It starts off with 1800 Attack and gains 500 at the start of each of your turns, and can't be destroyed in battle, or as a result of any Spells or Traps (meaning no Raigeki or Mirror Force will get it off the field). The catch is that it can only be Special Summoned with a Spell Card that you can only use if all five pieces of Exodia are in your graveyard. And if any of those pieces are removed from the graveyard, it is immediately destroyed and can't be revived. It can also be destroyed by monster effects (like Cyber Jar) and can be banished from the field.
The Dark Crisis booster pack brought quite a few of these in addition to Exodia Necross, an example being Berserk Dragon. While it has 3500 Attack, it can only be Special Summoned with the Quickplay Spell Card A Deal with Dark Ruler on a turn that one of your Level 8 monsters is sent to the Graveyard. It also gets to attack all monsters on your opponent's field once...but in addition, it loses 500 Attack at the end of each of your turns, which will quickly make it easy prey to anything tough your opponent has.
Consider its really cool looking and effect in the anime, Gandora the Dragon of Destruction is considered this for four reasons. First of all, this card cannot be Special Summoned, which means that you will have to tribute two monsters on your field (or Double Coston, since you just need to tribute it only). Secondly, you will have to pay half of your LP to nuke the field except this card. Although it banishes the cards, most players prefer Judgement Dragon or Demise, King of Armageddon as they have a much lower cost when nuking the field. Thirdly, this card gains 300 ATK for each card destroyed this way. However, the ATK boost is not impressive unless there are lots of cards on the field beforehand. Forth and lastly, this card is sent to the Graveyard during the End Phase of the turn it was Summoned.
Fusion monsters partially depend on what card you're summoning. In the early days of the card game, there were worthless fusion cards like Flame Ghost or Fusionist, possibly some of the most useless cards in the game. Other fusions that are useless unless you get very lucky or are very good are Dragon Master Knight, requiring Black Luster Soldier (a ritual monster) and Blue-Eyes Ultimate Dragon, another fusion monster. Generally anything that requires more than one fusion monster is probably more difficult to use than the average player would have patience for.
Reign-Beaux, Overlord of Dark World: He's the big daddy of the Dark World archetype (until they got a new big daddy), who gain effects when discarded by effects (but not costs) and gain better effects when discarded by an opponent's effect! This guy, however, needs to be discarded by an opponent's effect in order to do anything, and while there are ways to force an opponent's effect into letting you discard (such as with Dark Deal) they're way too inconsistent to rely on as a main tactic. But if you do manage to discard him by your opponent's effect, you get a monster with a respectable 2500 ATK AND you get to destroy all your opponent's monsters OR their spells and traps! However, Goldd, Wu-Lord of Dark World is only marginally less awesome but summons himself when discarded by your own effect, so he's one of the more practical cards in a Dark World deck.
With the publishing of the "Gates of the Underworld" Structure Deck, Dark World has become an extremely easy Archetype to exploit. Why? Because of a little Level 1 monster called Ceruli, Guru of Dark World. And what does he do? Simple. When he's discarded, he Special Summons himself to your opponent's side of the field in Attack position, then forces the opponent to discard one card of their choosing. Which in itself sounds harmless enough, until you realize that when he's Special Summoned to the opponent's side of the field, you become the "opponent" mentioned in the card effect because he's not under your control anymore. No wonder Dark World is considered one of the more idiot-proof Archetypes with cards like that. He effectively lets you turn your own discard-effect into your opponent's card effect at the cost of giving them a monster with (oh no!!!) 100 ATK.
Even if it has the second highest ATK points of any monster in the game, Machina Force is one of the best examples of Awesome but Impractical due to its extremely difficult summoning conditions of having Commander Covington on your field and sending Machina Defender, Machina Soldier, and Machina Sniper you control to the graveyard. To make things worse, it cannot declare an attack unless you pay 1000 Life Points. What's more, it's a "NOMI", meaning that if your opponent destroys it, it's gone for good (unless you feel like using something like Monster Reincarnation and repeating the whole process of summoning it again.) This card is even impractical in a deck that uses other Machina, especially since Machina Fortress, a far more practical boss-monster for the archetype, was released.
Sophia, Goddess of Rebirth, from the latest Duel Terminal. It has a heavy summoning cost of requiring a Ritual, Fusion, Synchro, and Xyz monster to be on the field. However, they can be on either side of the field, and can be tributed against the opponent's will a la Lava Golem. Once it's summoned, its effect activates which banishes everything from both players' fields, hands, and graveyards. Neither Sophia's effect nor its summon can be stopped, meaning that if you pull her off, you'll get an instant 3600 shot at your opponent's life points, if not a game win due to their loss of resources (unless they pull a card like Dark Hole or Mirror Force out of their ass, and even then, they likely won't be able to do anything other than that). Difficult to summon, by no means splashable, but in the right deck (and when you know your opponent enough to use his monster selection strengths against him), it can make quite a punch.
Back in the old days of the game, there was Gate Guardian, a 3750 ATK monster who can only be summoned by tributing his three components, each of which require two tributes themselves. The best bit? Gate Guardian's three pieces combined have twice as much ATK as Gate Guardian, and 3 monsters are harder to get rid of than a single target, so Gate Guardian is impractical even in a deck based around him.
The only real practical use of Gate Guardian is to fuse him into UFOroid Fighter, a fusion of the nearly useless UFOroid and any Warrior monster. UFOroid Fighter's attack is the sum of the monsters used to summon it, but if you use Power Bond to summon him, you'll double that attack. Using Gate Guardian, the strongest Warrior monster that can be fused, to make this card, you'll end up with a monster whose attack exceeds 8000.
Thenien the Great Sphinx: 6500 ATK on the turn that he's summoned, and 3500 ATK on all the other turns. In order to get him out, you need to summon Andro Sphinx, and Sphinx Teleia. Each requires two simple tributes to get out (unless you control Pyramid of Light - see below - in which case you can Special Summon them via their own effects by paying 500 Life Points apiece, but only from your hand). You then need to somehow destroy BOTH at once in order to summon Thenien. They do make a card for accomplishing this, called Pyramid of Light. When active, it does nothing, but if destroyed, it will destroy all of the Sphinx cards that you control. Problem is, if your opponent somehow destroys it while you have only a single Sphinx out, then you just lost your good monster.
The Spell card Reverse Quiz: To use it, you have to discard your entire hand and get rid of every card you have on the field. Afterwards, you have to guess what the card on the top of your deck is (Monster, Trap, or Spell). If you guess correctly, you get to swap Life Points with your opponent. Whilst this seems like a decent enough payoff, if you fail to guess the card on top of your deck, you're essentially defenseless against your opponent (unless you've got more Life Points than them, which defeats the purpose of playing the card in the first place). If you do guess it correctly, you'll still have to hope that the very next card you pick up is a decent card which can defend you from your opponent, since they'll essentially get a free shot at you after you activate Reverse Quiz. If it isn't a decent card, then by the time you can get a decent enough defense up, your opponent will probably have whittled your Life Points down to what they were when you played the card. In short, it is virtually impossible to play Reverse Quiz and get out of a situation which only it could have solved/improved.
Funnily enough there actually was a Reversal Quiz combo deck for a very short time: the object was to use a continuous trap card to reduce your life points to a thousand (such as Wall of Revealing Light which also disallowed monsters with ATK less than the amount of life points given up from attacking), then setting a combination of two kinds of equip spells (like Black Pendant and Fuhma Shuriken) that dealt damage to the opponent when destroyed. The trick was that because your deck was about 98% spell cards, a successful Reversal Quiz was almost guaranteed. Once you had Quiz'd, your opponent is left with your thousand life points, only to be burned for at least that much by the equip cards that Reversal Quiz destroyed. Resulting in instant victory. Unfortunately, because of the potential to abuse this sort of deck, Wall of Revealing Light is now a Limited card (meaning you can only use one per deck) making the entire strategy impractical.
The Wicked Eraser: First of all, he can't be Special Summoned, which means if he's sent to the Graveyard (which is quite likely considering almost all of his effects occur when he's sent to the Graveyard), he's done. (Unless you plan to use Monster Reincarnation, or something like that.) On top of that, his attack and defense are equal to the number of cards your opponent controls, which means if your opponent controls only one monster with 1000 ATK or more, he's done.
Some of the Spirit Monsters from the Legacy of Darkness booster pack and such can be this. An example is the Yamata Dragon, who packs 2600 Attack and upon inflicting battle damage, allows you to draw until you have five cards in your hand. There's also Hino-Kagu-Tsuchi, which has 2800 Attack, and, if it does battle damage to the opponent, they have to discard their entire hand during their next draw phase before they draw. The problem is that both of them take two tributes. Now let's look at the main drawbacks of Spirit Monsters. First, they can't be Special Summoned in any way, so it's usually not worth it unless you build a deck around them. Second, they return to your hand at the end of the turn, so it can mean trouble if they're the only things you have on the field. There is the Spiritual Energy Settle Machine, which allows them to stay on the field, but you have to discard one card from your hand each turn to keep it on the field. And if that leaves the field at any time, the spirit monsters return to the hands, so it's not the easiest thing to maintain.
As of The Duelist Genesis booster pack however, Spirit Monsters received some much needed support and are much more playable now. While most still cannot be special summoned, they received huge support in the form of Mirror of Yata (which allows them not to be sent to the hand, as well as serving as a shield- if the spirit monster were to be destroyed by battle the Mirror is destroyed instead) and Izanagi, a non-Spirit Monster who allows you to chose whether or not you want the monsters to return to the hand at all.
The Counter Trap card Judgment of Anubis has a cool name and an awesome effect: you can negate a card, destroy a monster, and inflict damage equal to its attack points all at once. However, the negated card in question has to be very specific: a Spell card that destroys your Spell/Trap cards. In practice, it becomes essentially an anti-Heavy Storm/Mystical Space Typhoon card similar to cards like White Hole which are powerless against anything else and thus are most likely to just be a dead draw.
Big Shield Gardna has 2600 defense, and it requires no tribute to summon. The catch? At the end of the damage step of a battle where this card is attacked, it turns to attack mode, so that you lose life points almost as if the attacking monster had just attacked your life points directly.
Number 88: Gimmick Puppet of Leo is an Xyz monster that has an effect that can allow you to win the duel within 3 turns of it being on the field. However, it requires you to have no spells or traps on the field to use its effect and keeps you from conducting your battle phase if you do. This is extremely dangerous against any skilled opponents since it allows them plenty of time to get the right card to stop it before you get that far and requires you to be very cautious about what spell/trap cards you set/activate lest you give up the ability to use its effect. And given it takes 3 Level 8 monsters to be on the field to summon it in the first place (Monsters that usually have very high stats), you'd most likely have a better chance of winning by just attacking with them and not having to be careful with placing or using traps/spells.
The biggest risk with this card, however? Your opponent can chain to the effect that would lead to its win condition with a card like "Enemy Controller" and take it from you...at which point the win condition goes off and grants them victory.
Some of the Archfiend monsters, particularly the ones introduced in Dark Crisis, can be considered this. On one hand, they have a side effect that allows you to roll a dice in response to your opponent's cards that target it, and if it ends up a certain number, you can negate the effect completely and destroy it. However, it comes at a rather large turnoff of a cost: they force you to pay between 500 and 900 Life Points (depending on the monster) during each of your Standby Phases whether you want to or not, unless you have less than the required amount (in which case the monster is destroyed), even if Skill Drain is active. You can remove this effect using the field card Pandemonium, however.
Rocket Arrow Express has 5000 attack (one of only 5 monsters that do) and just requires your field to be empty to special summon it from your hand. The problem is that it's effect makes it so you can't conduct your battle phase the turn it's summoned, you can't activate any card effects or set anything while it's on the field, and it destroys itself unless you discard your entire hand each of your standby phases. All of this adds up to a monster that has extremely high attack but makes you into a sitting duck for your opponent when you summon it (unless the opponent happens to have a Skill drain on the field...).