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.
Anticlimax Boss: A relatively minor villain in Xenos is shown to be an ungodly powerful psyker. Not only could he force the submission of a Chaos Space Marine, he was able to tear a cipher for the Necroteuch from the walls of the saruthi tetrascape. He forced a wall to divulge its secrets with the power of his mind. It left him largely burned out, though, too weak to stop Heldane from just shooting him.
At the end of Hereticus, a Daemonhost that belongs to Glaw was thought to be stronger than Cherubael by dint of being less stringently bound. Cherubael was apparently strong enough that he wiped the floor with it anyway, though we don't get to see it. He does make a note about being nastier than Eisenhorn ever imagined.
Badass Decay: Eisenhorn himself undergoes an in-universe version as he gets older. By the end he is running on fumes and sheer will.
Cherubael's threat to Eisenhorn and Bequin at the climax of Xenos firmly establishes him as someone utterly evil:
I will break every bone in your body and rape that girl by your side. And break every bone in her body. Then I will drag your jiggling carcasses down to the cathedral and burn out your agony centres while I wait for the bombardment to flatten this place
Pontius Glaw trying to kill all of Eisenhorn's old friends.