It is an extremely... epic story.
This is the only coherent critique I have ever seen: that it's very epic and mythic and high adventure, to the point that it reads more like a fantasy novel than a pseudo-realistic military history. This is probably why I like it more than the Ciaphas Cain novels, and why I like the Cain novels more than anything else I've read in the Black Library. But, some people prefer dry military-history style Space Marine novels - it comes from our hobby's history, I guess - though I can't see the appeal.