I am a huge fan of David Weber's writing. I doubt there are very many of his novels which I have not read, and I own copies of almost all of them. Yet it was a struggle, despite how much I've enjoyed the Safehold series, to push on through to the end of this one. It seemed to me that to "get" what was going on, you'd need a huge hex map of Safehold, and lots of little counters to represent each of the armies and their forces involved in the tale, as well as having your "cheat sheet" handy to keep track of the major players (though Weber does include a list of all characters at the back of the book) and what has happened in the past several novels to each of them.
The major threads of the story here seem to be, first, the plight of the Republic of Siddarmark and its people following the attack of the Sword of Scheuler which destroyed much of the food supply for the nation just before winter, as well as committing horrendous atrocities upon the "heretics" (aka Reformists). The Empire of Charis (Caleb, Sharleyan, Merlin and friends) is determined to do everything they can to get desperately needed relief supplies to the general populace and to reinforce the Siddarmarkian army with all the military personnel and supplies that they'll need to stop the imminent invasion by Mother Church.
Second, after being rescued from assassination by Clyntahn's agents, Princess Irys and Prince Daivyn of Corisand are welcomed by Caleb and Sharleyan back in Charis. Despite the fact that King Hector, their father, was an enemy of Charis, they are treated with the utmost respect, and it is hoped that they can be turned into Charisian allies in the near future.
The ongoing development of technology which doesn't technically violate the Proscriptions of the Church plays a major role here, as it has for most of the series, and Charis continues to enjoy a significant advantage in the machinery of war and commerce over its enemies, but the total numerical advantage of a great population and economy are difficult to overcome, especially when the Inquisition's spies are busily stealing the inventions nearly as fast as they are appearing. I suppose the suspense of the story couldn't be dragged out nearly as long if this didn't take place, but it's a bit annoying, you know?
Another annoying thing is that Weber will quite often introduce new characters, with a full backstory attached, and you never know whether to pay close attention to them, because they'll last for several books, or maybe the whole series, or whether they're going to just be around for this particular skirmish - dying horribly, or perhaps a few skirmishes, still dying horribly, but giving us a perspective on a bit of local strategy for a time.
For the most part, however, Weber's writing is engrossing, as always, and I'm still strongly committed to finding out what becomes of the Empire of Charis over the long haul.