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: Chosen Assaults. They're not only extremely tough, but their assault rifles have 100% accuracy and fire in three-shot bursts every two or three seconds. The only way to actually deal with them without losing half your health is to very, very carefully pick at them from long-distance, or make sure they're distracted by the local fauna before opening fire.
The Chosen, full stop. Their Assaults are nasty as it is, but Chosen Juggernauts running around with the same Plasma Cannon the player gets in their own Assault kit are even more of a hassle due to being a beefy enemy with a gun that can paste you in two good hits.
Hell Is That Noise: The Chosen have a weird Voice of the Legion, and their voices don't decrease in volume with distance. Therefore, as soon as you hear "The dawn of the Chosen is near..." you get the hell out of wherever you are.