The Horus Heresy novels, which got off to a fantastic, if never quite replicated, start with Dan Abnett's Horus Rising, reach, after a few stumbles, their first major fall with Fulgrim. The text is twice as long as it needs to be and about half as clever as it thinks it is. The gender politics, never the 40k universe's strongpoint in the first place, are particularly lacking in nuance which is a shame considering some of the arcs from the first few books.
Really, though, it's just a slog through turgid prose that fails to accept that the reason I'm reading a 40k book is for the violent hunks talking seriously about brotherhood and duty whilst they commit atrocities. 40k only works as a setting when there is a clear ironic detachment between the values of its protagonists and the values of those of us not mired in its horrible world. There are no good guys and, literally metaphysically, there cannot ever be any either.
Fulgrim spends too long trying to, in a mirror of the earlier books but without understanding what they were doing, show the fall from grace of its title character. Simultaneously it also spends too long detailing the kind of strategic machinations that would be better left to a few paragraphs in a codex book. What it needed to do was spend more time polishing the few action sequences it had, including sorting out the technical inaccuracies regarding Space Marine physiognomy, and maybe then it would have done Slaanesh proud.