The series began as The Breakfast Club in a post-apocalyptic world. In the third and final installment, Roth seems to have been going for a graduation from that high school prejudice into the prejudices of the adult (or "outside") world - racism. Except not racism, of course, but "genetically damaged" versus "genetically pure." But yeah, racism. Reading the book, as I am, so soon after Ferguson, I feel like it should resonate more than it does. 
 The "twist" fails, I think, because we've been peeling back the layers of the onion only to discover that it was actually an apple after all, and that just doesn't work in the third book in a trilogy. The idea of an outside world was set up early on, but the image we get of it as the big reveal at the end of the second book is shown to be a near-complete falsehood just a few chapters into the next book. As a result, if feels much more like a lack of planning than an actual twist. 
 There's also a rather significant stylistic change. Rather than all being in Tris's voice, the third book suddenly adds Four as a secondary POV character. Changing narrative styles so late in the game should be done with caution, and I don't feel that it worked here. 
 The characters seem addicted to revolution. It was one thing to fight against the Jeanine and perhaps the current fraction structure, that was set up early on and it made sense within the context of the book. I could even sort of understand a realization that the revolution didn't really improve things, and that the oppressed are often quite happy to become oppressors when given the chance. That would have made sense and would have made for a perfectly good moral lesson to tie the series together. What happens in the third book, however, is that Tris & co. first join the Allegiants to fight against Jeanine and the Factionless, then join the Genetically Damaged to fight against the Genetically Pure. They're so addicted to the process of revolution that they couldn't even stick with one for the entire length of the book. 
 (view spoiler)[The big sacrifice ending was silly. No one was in danger of dying, so Tris's big sacrifice was to keep people from having their memories erased, because she believed that erasing people's memories is a morally bad thing. Yet her sacrifice occurs while erasing people's memories. That's right, she believes that X is bad, so she sacrifices herself to do X. It's absurd. (hide spoiler)] 
 Overall, I found that the book felt rushed, and suffered from an apparent lack of planning throughout the writing process of the series. Through the other books, I was content with the peeling of onion layers. But in this one, knowing what the core looks like, really revealed the series's flaws.