It is in the nature of stories with fantastic setup to have less than astounding payoff, and The Stone Sky is perhaps hampered by the weight of expectation. There's a lot to love, here: fascinating backstory, funky worldbuilding, and satisfying resolution to two year old mysteries (in particular, the explanation of the second person narrator is really rather good). But in the end it was all just a bit shaky. The ending is too neat, Nassun's characterisation goes AWOL, arcs stall, plot holes are handwaved, info is dumped, the prose drops in quality from interesting to functional, and too much story relies on the minutiae of magical mechanics that aren't sufficiently explained to be properly satisfying. I'm sure that the book does what N.K. Jemisin wanted it to do, and it certainly functions as a satisfying conclusion, covering all the bases, but I feel like it needed another few months and an extra draft to mature and iron out all the kinks.