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.
YMMV: The Drop
Anticlimax Boss: The Bonus Boss, Tezkhra. With its tangential plot relevance and extreme rarity, you would expect it to put up more of a fight than the other "legendary" monsters. In truth, though, by the time you're likely to encounter it, you can easily kill it in a few turns.
Demonic Spiders: Filthy Mutts, and to a lesser extent, Territorial Curs. They're extremely observant, pretty strong, and surprisingly tough for their power level. Fortunately, they don't have any long-range attacks, but that doesn't count for much if they ambush you...
Fire used to be this in the demo, where it inflicted constant stun in addition to debilitating Damage Over Time. This effectively made it a death sentence, as enemies could almost always kill you before the fire died out, and there was nothing you could do about it. This was changed in the final version, thankfully.
The stun status effect in general, really. Anything that has a high probablitiy of inflicting it (like Stoneslide) can just be repeatedly cast until an enemy dies, and there's nothing they can do about it.
However, the flipside of this is that enemies have these spells too, so you can easily find yourself stunlocked to death as well.
Spells that inflict confusion are similarly powerful, since the status effect usually causes enemies to waste a turn.
Forced Purification. It's not quite as gamebreaking as the spells listed above since it doesn't inflict any status effects, but it makes up for it in sheer damage potential. It has a damage multiplier of 2.0, and deals additional damage to humanoids (which are often the toughest enemies) — against a noxious foe, it's almost always a guaranteed one-hit kill.
Goddamned Bats: Any enemy that is highly observant — you can't sneak up on them, they'll lock on to you at the drop of a hat, and once they do spot you, they will chase you to the ends of the earth. Dark Despoilers are particularly bad in this regard.
Jerkass Woobie: Yfus. He at first appears to be a haughty Deadpan Snarker, but it's actually a front to cover his crippling depression and loneliness after losing both his family and the guild.
Tons of people from across the world were snatched up from their homes and tossed into the chaotic monstrosity that is the Drop, full of slavers, looters, and monsters, as well as a construction that defies sense and logic. Some, like Sypak, managed to claw their way out. Others have been lost so long that they've gone insane.
The disturbing implications of trancing are cranked Up to Eleven in Tezkhra's ending. He is only at the bottom of the Drop for a short while, yet when he returns, the camp has long since been abandoned. Who knows how much time passed? For all he knows, all his friends are dead and their accomplishments undone — all in the blink of an eye to him.
The slimefly, scumflitter, and filmscuttler enemies. They're gigantic mosquitoes, and the game plays this up for all it's worth.
Slimeflies really want your blood, but it's not like they're rude about it! They're very enthusiastic about "keep[ing] it warm for you!"
The scumflitter's "Putrid Clot" ability: "The scumflitter spits a plug of rotten blood at you!" One of their favor items is some putrid blood, too...
The filmscuttler's "Deep Puncture" ability: "The filmscuttler buries its spear deep in your flesh!" Euugghh.
Tear Jerker: Yfus' campaign in general. Unlike the rest of Six Stars (the ones we see in-game, anyway), he was never able to recover from Havan devastating the world, and the members of Six Stars going their separate ways only made his depression worse. By the time his campaign begins, he's become a bitter, jaded shell of the Hot-BloodedNice Guy he was in The Reconstruction. Even after the Tatzylvurm tries to give him advice on how to improve his life and reforge the friendships he so desires, the ending heavily implies that the Tatzylvurm's words actually made Yfus's depression worse and only served to hasten his downward spiral.
That One Attack: The Sanguine Soarer's Shocking Dive attack. It's quite powerful, has a long range, and has a chance to stun, which is usually a death sentence against such a powerful enemy.