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.
Game Breaker - the Wasp Hive series of rifles early in the game. Fire and forget indeed. "Skillful/Proficient" gear may apply as well, as you can get high-level skills early on - such as Afterlife (which basically turns corpses into health potions) for the Evoker. Or you can get one of your existing skills upgraded a notch or two having a "rank" higher than maximum for that skill.
Goddamn Bats: Ravagers hop around in packs, usually scattered within barking distance of each other, all over the map, and are worth paltry experience for your troubles. The Chompers may be this to some, but to some they're kind of cute — until they get bigger.
Closer to the trope, the Shriekers fly around on bat wings, shrieking and nipping, and also fly in packs. They're harmless unless they're ignored, in which case they can chew a character to pieces.
The Goddamnedest is actually called the Schism Bat. A miniature version of the Warper, they're easy to overlook and hard to target, and can quickly give you a status effect that makes you even more vulnerable to them. A pack of them is as dangerous as anything in Hell.
Tortured Souls (and derived enemies). These buggers may not have much HP, but they teleport around with a purple beam, which damages you. And they rarely miss... and tend to come in numbers.
Nightmare Fuel: The statues in the British Museum look creepy already. But then they come alive.
Scrappy Level: Embankment Redoubt, Sudden Genre Shift puts the player in charge of a Redshirt Army, to be torn up on the battlefield in situations players could, with the skills developed up to that point, handle by themselves.
It's not that bad, actually - you get to restart as many times as you please, and enemies don't respawn or regenerate. You also get to heal your troops and deploy an airstrike with a decent use rate.
Angel Passage is a major offender. You have to clean up a whole level. With a fixed weapon. And every destroyed Boil leaves a Poison damage field. You're not required to remove all of the Boils, but it's a good idea as you'll be facing a boss in a field full of them otherwise. You'll die fast.