What you've got to take in mind is that the warhammer world is limited to a single planet in the 40k galaxy, and nids only managed to spread themselves to a part of the galaxy.

Or perhaps they are there, but have "gone native" and can't be distinguished from all the natural crazy fauna. The catchan devil for example is a nid that ended up fully adapting to the catchan's homeworld.


Quote Originally Posted by Tyndmyr View Post
Back then, fantasy/40k crossovers were pretty common, and the view that fantasy was a world in the 40k universe was quite normal. I don't know if that's still the case.
The latest Warhammer fantasy codexes fully support it. Four chaos gods and all their daemons, name by name and aspect? Check. Old ones that ruled the universe long ago and traveled the stars and engineered stuff all around? Check. Humans who serve chaos geting super armor/weapons and powers and eventually ascending to daemon princes or spawns, check. Psykers/sorcerors who draw energy from the warp and risk blowing up their own brains, check. Orc/ks that can make the impossible happen just by believing hard enough on it and with grots that have an uncanny talent for making machines, check.