My main problem with this book... you know how every writing class ever will include "Show, don't tell"? This book is completely full of the reader being told of vital story elements that we would have REALLY liked to have seen rather than be told about in a single throwaway line. Instead of focusing on the characters and the story, it's going through breakneck speeds through anything that vaguely resembles those things to focus on scenery descriptions and battles.
It's actually made worse by the white-on-black shorts that show what could have been. Guilliman's conversation with Yvraine when she's leaving is half a page of goodness that shows an understanding has grown between them - which we've seen nothing of beyond that. Similar for the triumvirate of the Imperium, all three of those around from Fall of Cadia to the end of Rise of a Primarch. While it's obvious they interact a lot in two of these books (with Guilliman added to it in Rise), we don't get to SEE much of it.
The cast is fantastic, the script they got to work with pretty horrible.
Ask not the Eldar a question, for they will give you three answers; all of which are true and horrifying to know.