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.
Demonic Spiders: Crusaders, giant robots with triple rocket launchers and flamethrowers for close range. They take a lot of hits to kill, and their shots travel very fast, but what puts them in Demonic Spider territory is that they don't telegraph their attacks. All shots are fired near instantaneously, giving you very little time to get out of the way.
Disappointing Last Level : The first 5/6ths of the game are a good balance between combat, exploration, and character interaction/story advancement. Despite being a Doom engine game, it's all very RPG-like. Once you reach the Loremaster's factory, however, the game pretty much turns into a couple hours of straight Doom-style combat through several levels of identical corridors, leading up to the final battle. You can skip the Loremaster's factory based on the choice you make at the decisive point in the game, but you end up with the less good ending as a result.
That One Level: Dear gods, the Training Facility. You need to complete it in order to get full stamina and accuracy (and, in the Steamed re-release version, to get the "Top of the class" and "Fully Amphed" achievements), but doing so is virtually impossible; you have to defeat a Crusader at ridiculously close range, and if you survive that you have to (several times) clear death pits in the floor which can just be leapt over with a running jump, and then only with perfect timing.
The Proving Grounds runs a close second for being so confusing. Try to get through it without consulting a walkthrough, or examining it with a level editor.