I think most people that dislike the novel, (though they wont admit it), because it shows that the Thousand Sons weren't as innocent as they we were led to be in Graham McNeill's novel. But even if you read his novel, there are points in there that point out at Magnus being a fricken asshole before the rubric.
Disagreed. I think the likely reason for most people not liking it would be because their expectations hadn't been met. The blurb, the pre-book chit-chat does not lead you to think you're in for a sedate, cerebral mystery. Even a very well written, interesting one is going to be a bit of a let-down, to say the least, if you're coming for an action-adventure about destroying the planet of sorcerers whom the protaganists were finally being vindicated against...but all is not what it seems (but it's still an action-adventure).
Hell, I'm pretty certain it wasn't people crying about their precious Thousand Sons being beaten up by nasty Wolves.
As a further note, I think that the novel did extremely well in preserving the mystery of the Wolves of Fenris, by giving an outside perspective. I think they purposely did that, in a way to show, that there was no real way to understand how the Wolves worked.
I think they certainly did cultivate the Fenrisian mystery very well here. Though I disagree on the second point: I felt Wraight did this exceptionally well in Battle of the Fang
which makes a fine 'third book in the trilogy' combined with PB & ATS.
I don't think it would have been possible to write how one of the wovles would be thinking during a battle scene.
Again, I felt BotF did it rather well (without being 'too far' from the Heresy-era, indeed, many of the themes were precisely the same ones that could be carried over from PB/ATS, albeit separated by a while and with much 'left for other stories').