I think /u/_ereshkigal made a good point in his analysis when the book came out:
Well, I have highly mixed feelings about Battle Magic. While it was a good story and I always love Tamora Pierces books, it didn't quite meet my expectations and it added things to the Emelan Universe canon that I felt were better left vague.
Firstly, after reading the allusions to his experience in The Will of the Empress, I had built the war up to be some shadowy, horrific thing. Reading the actual scenes felt like a let down. The fighting and gore was hardly gritty and explicit enough to elicit that visceral reaction I had come to expect. Yes the things described were terrible and would understandably scar anyone, but they did not match what I had pictured in my mind. This is the first time that I have had to remind myself while reading her books that Tamora Pierce's audience is primarily young adult, and I am moving out of that demographic. My second issue was that the gods are now 100% confirmed real, while in previous books they had been prayed to and worshipped as beings which may or may not exist. I liked that in Emelan the gods did not speak to the protagonists through dreams or in person. As characters and plot devices in Tortall they work well, and I enjoy when they enhance the abilities of their champions since that was part of the canon from the very beginning in Alanna and In the Hand of the Goddess. However, there they had limitations and could only really affect the world through their chosen. I found the gods' interventions in Battle Magic a bit distasteful because it was too much of a deus ex machina. Actually, it was pretty much the exact definition of a deus ex machina.
But I did like learning more about Rosethorn's relationship with Lark, and the God-King was a cool character. I liked how brave Evvy was, and it gave continuity to Melting Stones and made me forgive her for some of her more callous words. I felt that Soudamini and Parahan could have been a bit more developed, but they were good characters nonetheless.