I was fine with most of the ending.
I had no issues with Rachel dying. I thought the notion of everyone getting through was completely unrealistic and it wouldn't feel right if the war ended without a personal sacrifice of some sort. Rachel, being the one most clearly and absolutely defined by the war, seemed the right choice.
I had no issues with post war drama with respect to Jake and Tobias, either. Anyone who expected this to end gloriously given the path we took simply viewed the series in a much different way than I did. This war was anything but glorious and always ambiguous. Why would anyone expect it to end differently?
As for the Jake/Cassie relationship, I dunno. Maybe I simply didn't have as much invested in that as some other people did. I wasn't upset.
As for the ending with The One: Well, I said I was fine with ALMOST all of the ending. Sure, I get that war sometime leads to more war. But I can't see any reasonable rationale for introducing a brand new character in the finale pages of the book, providing almost no explanation for its existence or nature, and having it be the core of the "endgame" for this book. That was jaw dropping.
Overall, I wasn't nearly as put off by the finale as others were. The only part of it that really, really bothered me was The One.