You know I read sometimes on this forum that Comissar Cain books are not reliable source and so on. So topic title question arises.
What's wrong with it? We haven't some kind of verified canon, and I can point an official book saying that all Marines are united under one man command, Imperator should eat souls to live but is able to saying and commanding, and Imperial governments are tolerant to Eldar adventurers who runs to Imperium leaving their dull Craftworlds. I belive first edition is not valid now though.
But can somebody point what's wrong with Cain books? Can't say I'm using them as primary source, but I can't point anything definitly controversal there.