All of which is, of course, completely and utterly true. But with the aliens, the effect they have on the wider game is vastly different. A game featuring the Tyranids is going to feel vastly different to a game featuring the Eldar, which in turn is going to feel vastly different to the Kroot, or the Xenarch or the Enslavers. Whilst you might be able to play a fun game of 'spot the b-movie Games Workshop stole these boys from,' the adventures and stories will feel very much different to one another.
Whereas plague zombie survival horror, genetically created zombie survival horror and warp-spawned zombie survival horror are, when push comes to shove, all zombie survival horror scenarios. I like zombie survival horror, and feel that World War Z and Pride & Prejudice & Zombies should be required reading in schools from the age of 4+, but Dark Heresy is getting a bit - dare I say it - infested...
The grass is always greener...