This book starts off centered around side-characters from the previous books: Orbus, Sniper, and Vrell. And the plot doesn't even touch the planet of Spatterjay itself. I wouldn't mind that in itself; actually, the first parts of the book have shown interesting character development for Orbus. That's something this series really could use.
But by the halfway mark, all of that was forgotten... and also the book pivoted from a nice sci-fi to a sci-fantasy with elements I also disliked in Asher's book Dark Intelligence. It's not all bad; even though the descriptions of battles were long and drawn out, the rest of the plot was okay, and the ending was good. However:
Without going into spoilers, what basically happened was they resurrected the Sealed Evil in a Can, which immediately had random godlike powers and got to ignore all the rules of the series so far. As a narrative device to heighten the stakes, it failed for me utterly. If it were handled differently and had the character development not disappeared in the latter half of the book, this might have been the best book of the series. (Most especially, if the time scale the Jain thing happened on was longer; then it could have been written without what basically boils down to "suddenly the balrog got resurrected and did evil magic".)
As it is, it's the worst one by a wide margin.