These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Fire Elementals, and to a lesser extent their smaller cousins. They're level 50, the highest the player can go, insanely tough (because they're made of fire), damage both your armor and weapons (again, fire), and have a nasty habit of appearing in packs. One quest actually forces you to trek through a cave full of these things.
Earth Elementals are this to a lesser extent, especially since you're forced to fight them a lot earlier than you'd normally be ready unless you abuse the mechanics to jump ahead 30 levels. Like Fire Elementals, attacking them damages your weapons since they're made of rock. Some of them also come superheated for fire damage, making them almost as bad as Fire Elementals.
Finally, at the bottom of the barrel but still no less annoying, are the blue zombies. Not only are they at least twice as tough as any other zombie, they cause fire damage when they strike. Sensing a pattern here?
In an example that surprisingly doesn't involve fire, the barbarians at Kree are some of the toughest human enemies in the game, easily equal to a late-game melee character. To make it worse, it's extremely difficult to attack them without getting mobbed by several at once. A resurrect spell or tech-based alternative is highly recommended, and they are prime zombification targets for the same reason.
Designated Villain: Z'an Alurin is canonically evil. She does not act that way; at worst, she's a very detached and philosophical sort, she acts like a Defector from Decadence, and we see her commit no evil actions (unless she's participating in the player's villainy).
It is possible to create a character who creates explosives, scours the streets of Tarant, and crafts random and very, VERY profitable parts from them, trading up as their skills progress. Repeat ad nauseum, and you'll have a character who levels a warehouse at the slightest bump.
Similarly, if you go the magic route and specialize in Force magic, you will eventually be able to cast the Disintegrate spell at half endurance cost. It will completely and instantly vaporize whatever it hits, essentially letting you run all over the place destroying anything that stands in your way, be it monster, NPC, or a door. It destroys loot, which is something of a downside, but there are a lot of things to kill that do not drop loot (or drop worthless loot).
The fifth-level time magic spell tempus fugit speeds your party by a factor of two, and slows everything not in your party by the same factor (no saving throw, no immunity, magic resistance does not apply). It has a large up-front cost, but the cost to sustain it is trivial. It's every bit as broken as you would expect for something that increases the number of actions your party gets per enemy action by a factor of 4.
Fatigue slowers halve the fatigue cost of all spells while they're active. Combine that with fatigue-restoring potions, and you can throw around several hundred fatigue points worth of spells without rest. And yes, that can be combined with Force mastery, which allows you to Disintegrate the entire population of The Very Definitely Final Dungeon.
Balanced swords can be created in the first town or have Magnus make them for you by the second. They have one of the the fastest attack rates in the game, but still have good damage. Note, however, that they are tech items and good mages (single-college mages like Virgil will manage just fine) will probably kill themselves trying to use them. A step up from this is the featherweight axe, which is even better. The axe can then be upgraded into the deadly pyrotechic axe, which pound for pound is pretty much the deadliest tech melee weapon in the game thanks to fire being such a broken damage aspect. The only things that outclass it are Infinity+1 Sword weapons.
A minor one for technologists is the electric ring in the second level of the electric tech tree. It gives a +2 to Dexterity when worn. You can wear two. In a game with only 60ish character points to give your character, four extra is a huge bonus, minus the two you spend to get there. If you're going for electric mastery, then it might as well be four more points.
The dog. Really high damage, fast attacks, and usually hits. For added cheapness, he even automatically gets the mastery bonuses that you have to either pay or work your ass off for. Makes combat a bit too easy, and kill-steals like you wouldn't believe.
He can even bite open doors and chests inflicting only minor damage to himself. You do not need keys or unlocking skills/magic anymore.
Every combat skill has a matching uber-weapon, almost all of which become available at an appropriate stage of the game. However, Throwing specialists can obtain the Aerial Decapitator around halfway. Finding it may be a Guide Dang It, but after that you have a one-hit killer that requires no ammo and doesn't break. It's also one of the fastest weapons, so even if the first hit isn't lethal, your opponent probably won't be able to retaliate before the next one does the job.
Even though it's somewhat late-game, the healing mechanical spider gifted to the PC by returning the camera from the crash site counts. As soon as you have it, forget any possible need for any type of healing. One spider, so long as you can keep your party alive, will heal anything short of a maxed-out mage.
With just a few points invested in Pickpocket (or Save Scumming), you can steal a key from Ristezze, a merchant in the first town, allowing you to open his loot chest and take his entire inventory (It's mostly junk, but technologists can craft some of it into useful items) and all of his money (usually about 1300 coins), and continue to do so every time you visit Shrouded Hills for the rest of the game. Never again will you worry about money.
Once you find the Necromizer schematics, you can turn some of the most dangerous enemies in the game into your eternal zombie companions. And if they die, just revive them again. The ingredients can be bought or stolen from shops, so you craft an endless amount, for free if you have the lockpicking skills to rob them.
Genius Bonus: You shouldn't be surprised that the Zephyr is doomed once you see its interior: the staircase and lounge look just like those on board the Titanic.
Good Bad Bugs: The Reflective Shield spell has a certain unorthodox application. Normally it reflects any spell cast at the character with the shield on them back at the caster, this goes for both beneficial and damaging spells. However, the shield treats canceling sustained spells as a spell in and of itself. Meaning if you cast, for instance, Polymorph on someone, then cast Reflective Shield on them and then cancel the Polymorph, then the cancellation will be reflected back at you, the Polymorph effect will be removed from your sustained effects bar but will not be removed from the target, meaning they are now permanently a sheep. This works for any spell that needs to be sustained, including Mind Control and summoning creatures.
Hilarious in Hindsight: Gilbert Bates, the presumed inventor of the steam engine, who holds the monopoly on making steam engines, is a reference to Bill Gates of Microsoft. His competitor, Cedric Appleby (probably an Expy of Steve Jobs), is a reference to the Apple corporation, who competes with Microsoft. Today, Apple has already surpassed Microsoft as the bigger tech company made famous by their I-Products such as the iPod, iPhone, and iPad.
With a market value of about $460 billion, Apple is worth more than Google, Goldman Sachs, General Motors, Ford, Starbucks and Boeing combined. – Apple is now worth almost twice as much as Microsoft (about $258 billion) and more than twice as much as Google ($198 billion).
Moral Event Horizon: Stout became captain of the guard purely so he could stalk a noblewoman, and when he found out she was already engaged, he rendered the fiancee blind in what was promised to be a fair duel, then arranged for the fiancee to become trapped in a cave full of monsters solely so he could save her (or have the player save her on his behalf).
That One Attack: As noted in Demonic Spiders above, anything with the potential for fire damage is immediately at least twice as dangerous as any other form of enemy. Fire damages equipment, most things are not resistant to it, and the enemies that tend to use it are also usually much tougher than other enemies. Fortunately, if you get your hands on something with fire damage, like the pyrotechnic axe, it's just as broken in your hands.
That One Level: The Black Mountain Clan Mines. It's the only part of the game you can't talk your way around. Moreover, it's filled with rock golems which break most weapons when you attack them and do insane damage compared to what you've been fighting thus far. Any combat-heavy area counts, really, but most of them are placed fairly. The Black Mountain Clan Mines are more notorious because they are a relatively early part of the main quest and because you can't get around doing them.
To a smaller extent, getting out of the first town can be this for the violence-inclined and/or those without the right build. In order to leave, you have to get past the guys guarding the bridge. Unfortunately, the leader has two half-ogres who will beat your party to death in short order. You get stun grenades early on, but without throwing skill you have to save scum for a solid hit. To get past charitably, you either need a ton of money (more than you're likely to have or want to spend), to sabotage the new bridge being built (which will make the town hate you), or a single skill point in persuasion.
Drop some railroad spikes next to the thugs. Because they're all using their fists (which do a crapload of damage), the game considers them to be unarmed. That means they will automatically pick up and equip any weapon on the ground nearby, even if that weapon does far less damage than they'd normally do unarmed. Railroad spikes have a maximum damage of 1. Your team should be able to beat them now. The point is still good, though: without knowing how to take advantage of the game mechanics like that, that's an early-level Beef Gate that has put a fair number of people off from playing the game any further.
T'sen Ang if you choose to free the half ogre slaves. Everybody will turn hostile and they can be quite difficult.
At least Kree is optional, but the barbarians there are very fast, hit hard and are often in groups. You'd better bring along a necromizer or two.
The Woobie: Randver Thunder Stone loses his father to exile long before he's ready to inherit the throne, and he gets no endings that aren't at least somewhat bittersweet: either his father returns to the throne and he isn't even mentioned in the ending, his father dies heroically and allows him to become a good ruler, or his failures as a king lead to a civil war that either causes him to lose the throne to a stronger candidate, or drives the dwarven race to near-extinction.
Woobie, Destroyer of Worlds: Poor Torian Kel. After his resurrection he's on a hopeless quest trying to find a way to release his trapped friends. But what makes him a Destroyer of Worlds? Besides being evil aligned, he's willing to side with Kerghan's quest in exterminating all life so said friends can be released, making him a literal example of this trope.