The background of 40K is vast. And it encompasses 38.000 years. There is a saying about it, and if I remember correctly, it comes from one of the bigwigs at GW:
Entry: Hydra Legion.
That leads me to another prejudice I want to talk about: “Fluff Bunnies are less competitive and soft targets”. Well, in my humble opinion that is not always true. Yes, there are fluffy armies that would need a lot of luck to stand up to an optimized tournament list. But usually a fluffy army is not at a big disadvantage. An all-jump pack “Host of Angels” Blood Angel army is themed, fluffy and capable of wrecking carnage. Or what about a Tzeentch-only daemon host?
And even if it is something that contradicts canon: