It's kinda sad. The first book, Priests of Mars, was the best. The second was a bit worse. And this final volume is the shittiest of the three. It's still a good book in the sense that it's a nice read, there's some action going on, some disturbing things happen to some main characters and all in all the book gives some closure to the series. But still.
After three books, the techno-nonsense becomes stupid. It's all fun to get you into the atmosphere of a techno-cult in the 41st millenium, but if half the book becomes gibberish, you have to ask what you are spending your time on. The enemies in the book are flat characters, soulless nameless crystal-things that just spawn and die. The main villain has no depth, he just went "insane".
There are some satisfying sub-plots in the book (I appreciated Lydia and Galatea, and some dynamics with the Black Templars) but most of it is pretty basic. It doesn't help that this book isn't that big on interpersonal relationships. The first book took it slow, it was all about fleshing out the characters. This book is the opposite, it's pretty much one big action-packed battle with characters I couldn't care less about. Especially the Cadians. I don't know why they got such a big focus in the book, possibly because they are "baseline" humans and thus supposed to be most relatable? I couldn't find myself in their hero-complex at all. "We're from Cadia so of course we're super soldiers, we never give up, Cadian soldiers are the best. Btw, did we tell you we're Cadian?" Give me a break.
Perhaps after three books the Adeptus Mechanicus doesn't feel special anymore. In some way the trilogy of Mars reminds me of Game of Thrones. It starts of amazing, but then the ending is just some generic action thing that leaves you unsatisfied. It's still a nice read, but nothing special.