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: Word Bearers
Fridge Logic: In the third book, the Undying One starts using the Nexus Arrangement to cut off the Warp influence around the planet. Teleportation throws people through the Warp. Oops.
Magnificent Bastard: Erebus single-handedly caused a war between two civilizations, leading to the complete annihilation of one, stole the most powerful artifact of one of those races, orchestrated the downfall of Humanity's greatest protector, caused a galaxy spanning civil war that almost destroyed the Imperium and is indirectly responsible for half the Space Marine legions converting to the worship of Chaos.
Nightmare Fuel: All the descriptions of Varnus' predicaments are this.