This is more of a review of the series than the final book. Spoilers ahead.
There’s two hurdles with steampunk fiction: the gadgets (and to some extent, the fashion), and the alternate history. The former challenge is a highly visual concern and doesn’t lend itself immediately to the prose format; it’s more suited for a cinematic or graphic novel treatment. Mayer does a decent job of explaining the gadgetry without getting too technical for the casual reader, although an image search made clear some of the more mechanical terms (a “knurl” wasn’t what I thought it was). Writers are almost better off not including illustrations of the alt-tech and leave it up to the reader’s imagination instead. There were a few times where the mental model I formed of a certain gadget or costume turned out to be depicted on the cover or on one of, and it was almost a total mismatch.
The second hurdle of alternate history is more thematic. Newer writers will miss the mark on this badly because they can’t resist stuffing 21st century beliefs and words into the heads and mouths of the cast of characters. What makes it worse is when the and none of the other cast share the same views, so there’s little plausibility for characters that break from the type, and because these breakaway characters have opinions that are acceptable to modern readers, they pass notice. Mayer’s cast is stuffed to its leather goggles with 1.5 dimensional ethnic and cultural stereotypes: aristocratic Anglos, emotional Italians, the anger-issue Irishman, the intelligent Jew, the racist Southerner, the dopey German (Prussian) with a thick accent. You have to admire an author who looks at the entire buffet of ethnic tropes and just piles just about all of them onto his plate. The two character aberrations from the period are the smart and spunky (thankfully, not sassy) suffragette protag and the possibly noble post-Civil War ex-slave. The armies of heaven can only help Mayer if he didn’t make those two the most sympathetic characters.
A reader could see some of the conclusion points easily. After all the legacy Euro men were literally killed off, the final cast was the lovingly checkboxed list of diversity you would expect; an appropriate modern storytelling denouement, although Mayer missed the boat with the wheelchair guy. Other aspects of the ending were better. I didn’t predict accurately the fate of two characters, the Italian woman and the alcoholic do-nothing frat guy, two of the more interesting personalities Mayer provided. I prefer Clarke-ian, far-reaching consequences with my sci-fi, and the very last page provided this in a nicely poetic fashion.