This book has some good points; it is well written generally, and the characters are well drawn, being interesting, varied, and quite detailed. However where it falls down is the plot, which sadly feels very slow moving, dull, and padded out - especially for the last third of the book. There is also a lack of villains, with no real bad guys to spice things up.
I do think that McNeill is nobly trying to do something new with this one, which I do like to see, telling a story that is far less about 'only war' than is normal for 40K. Bolter porn this is not. However, the story he decides to tell instead just does not feel appropriate for 40K, based as it is around something that is essentially a more powerful version of the Genesis Device from the Star Trek movie 'Wrath of Khan'. Indeed, even before I twigged that similarity I felt this book was rather too Star-Trek-like for a 40K novel.
I tend to think this trilogy's story could have been better told across two novels rather than three, as the thought of a whole further novel to wrap things up is quite a daunting prospect.