This was a fun conclusion to this trilogy.
I think ultimately, this is a book driven by the principal point of view characters and their actions. Despite that many of them are leaders of empires, islands, or organizations, this fantasy series is less about political maneuvering and more about interpersonal conflicts and shifts. This made a lot of sense in book one, following three or four different characters in different places, none of whom were in real positions of power. However, I think I was a little surprised at how much this style continued through the whole series.
In some ways there's some superhero vibes, where the sides are determined by which heroes are opposing or supporting whom at any moment. I said this about book two, but I appreciated the ongoing shifting allegiances. Characters join up, and then split apart and join opposite sides. Some are forced to be on sides they don't want. Some help each other only reluctantly. I think it works really well and creates some good character growth moments.
In the end, I still feel a little like some of the magical world-building details got handwaved through more than I wanted. I'm still not sure I really understand the underlying reason why islands are sinking. I understand what actions make them sink, but not why. (Maybe I don't understand what makes them float in the first place?? ) I feel like the memory pools also felt a little too convenient, and I wish this were better foreshadowed or explained. There's some "I hope you don't find out what this magical artifact really does", but then that detail is never revealed. The book also itself asks (but does not answer) what the connection is between all the things that bone shard magic can be used on.
I'm not trying to nitpick details here and I certainly don't need some intricately built magic system in a book, but it just didn't all quite hang together for me in a way that I wanted.