Just finished a couple of days ago. Overall, I'd give the book a 6/10. Not great, but not bad.
Some points for consideration;
Maybe it's a retcon I missed, but I got the impression from DL that all Chaplains are from the Word Bearers.
I thought Chaplains were a newly created, internal office, with the incumbent being selected from within the legion.
The Word Bearer's execution was pointless, and I was rankled by Corax's newfound "Chaos bloodhound" ability to sense Chaotic taint. I'd rather we discovered later that the Chaplain was actually innocent.
Nice to see the Custodes growling at Corax for questioning Nikaea; that combined with the oath the Emperor made him swear put the Thousand Sons and Magnus in a new light for me.
The puzzle sequence was ridiculous. IIRC, the 'solution' could just as easily have been to throw enough bodies at the complex to confuse it. A huge security risk.
Huge sigh of relief with the Alpha Legion; couple of plot points in there could/should have been the epilogue for Legion - not all the AL really know why the Legion betrayed the Emperor, moving the image away from them all being covert loyalists.
I was genuinely surprised to discover that Solaro was one of the operatives. I expected it to be Agapito long before there were any hints dropped in DL; something about his description at the end of Raven's Flight.
The assault on the Perfect Fortress read more like an epilogue; the book could just as easily have ended before that. I'm not complaining, mind, just saying...