I'm not going to complain, as others have, that they mixed Fantasy and SF in the same book; that's fine.
But what others call a "refreshing twist" is, IMHO, committing one of the cardinal sins of poor writing. A good author resolves a plot - characters' motivations get worked out, events progress to some natural conclusion, happy, sad, neutral, disturbing, or otherwise. Questions, at least some of them, get answered.
Weak authors build up piles of tension and intrigue then discover they've planned no way out. So instead of resolving plots and characters, they bring in new material that simply makes the previous thousand pages irrelevant.
Nations stand at the brink of war. Our hero is caught and conflicted, feeling responsible for the impending havoc. Each side thinks the other posesses the secret weapon. The motivations, hopes, and dreams of a few dozen supporting characters are balanced on the proverbial knife. The reader is on seats' edge, wondering which noble hearts will receive an unjust end, and whether villains will prosper or bleed out on the end of a white knight's sword.
When all of a sudden, a nameless, faceless force (farce?) with no logical motivation and with whom the reader has no relationship shows up and slaughters everyone. Can't figure out how to resolve that military hero, dear author? One blink of a tank's laser iris, and your conundrum vanishes - along with your reader's sense of resolution.
The series' most entertaining character and biggest enigma, Simkin, simply vanishes, with no explanation whatsoever except for some unsatisfactory handwaving in a weak and aimless epilogue.
Fun story, interesting characters throughout the trilogy. The ending, ashes and emptiness, sans denouement.