The best way to sum up my feelings about this book (and this series as a whole, really) is that nothing feels developed enough. It's like a cake. The core elements - the flour, sugar, eggs, etc. - are there, and they're good, but it wasn't put in the oven long enough so everything about it seems underbaked. Like you're about to sneeze and you know it's gonna be a good one, but it never comes. Or a 2D drawing that was meant to be a 3D rendering. 

In terms of specifics, the ending action was pretty good.

Although the most important chapter in the whole book, where Katherine changes allegiance and Mirabella dies, came across as a sketch or an outline, so didn't feel like the momentous event that it was. Mira dying at this point before she could really made an impact also just turned her into a non-character for me.

But apart from that, I don't have much else to say.