TLDR: Weak plot, little to no character development.
A promising setting and cast of good potential characters, followed by an underwhelming act 2 & 3 as well as little to no character development. it's a blockbuster action novel with subpar action and story.
Spoilers to follow:
There are 3 main loyalist and 2 main traitor characters followed throughout the novel. Each act in a cookie cutter predictably and the author has seemingly forgotten that it's the human quirks and characteristics that make the space marines alluring. It's their seeking perfection whilst maintaining a link to humanity, that makes them relatable and interesting. As listening, I kept waiting hoping the story would get better but instead I felt I spent 10 hours listening to the tale of 5 servitors disguised as Astartes- How underwhelming. Moreso, the big boss battles at the end of the book were incredibly disappointing whether the fights were finished in moments to an outsider, off screen or worst of all in a way that did nothing to credit the characters previous personalities or arcs (think Denarys end of GOT).
I believe no storyboard was made when writing this up. And no plot arc. The author references religion as a reason one main character gives up but there's no build up to this. It's just a casual shrug that makes NO SENSE at all. A word bearer saying "oh well" the gods don't will it anymore.
All in all it's another brick in the servitor wall of bad Astartes-books. Do yourself a favor; avoid this one.
PS. In light of not trying to be overly critical: perhaps the author was rushed for time on this one's execution? Budget causing a rushed product?