I feel like this one didn't pay off as much as it should have and it was a lot slower than the two before it. It felt very cyclical and relied heavily on the will-they-won't-they stuff which became very repetitive and boring.
I also didn't really see the point in most of the POVs of the Ezra Six. They didn't seem to add anything to the plot or the main characters development. Some of them added a little bit, like the last one, but mostly they were unnecessary.
At the start of this series, I thought everyone was either going to die or create a polycule between all six of them and maybe, if the author was brave enough, have a six-way polyamorous relationship. It seems I was kind of correct on both.

She was not brave enough for the polyamory unfortunately but the polycule was very real and though she did kill off some of the characters, she did it in such a way that it somehow completely ruined any development the remaining characters had. 
I kind of wish they had gone through with the experiment so that they could learn from it and realize how wrong it was or maybe realize they were right to do it. But as the ending currently sits, it feels more like Libby made the choice for everyone and no one really got any growth from that.
I couldn't even really tell how they all reacted to Nico's death. All of their reactions to it felt stagnant especially since he was never mentioned as one of the choices to possibly kill when they needed to make that choice. He was never a consideration for any of them, so the fact that all of them seemed to take the news of his death as a "oh that happened" kind of thing was weird. Gideon had the biggest reaction, naturally, but of the original six, I think the only one that had any big reaction to it was Reina and that was more or less just a realization that she would never get to tell him that she forgave him. She seemed to be the only one that would be affected in the long run.
Libby has to deal with the fallout of killing her "other half" but we don't get to see how that affects her at all. She runs straight to her ex girlfriend's death bed to try and get closure with her instead and then her story was just over.
And then Callum dies and we get Tristan going through a thousand scenarios in his head but ultimately being very blasé about it which makes no sense at all. I wanted to see him react, explode even. I felt like that would have been true to his character, but he just does nothing. I'm not even sure if he actually ended up killing his father in the end or not.

I don't know, this book just felt not as good as the other two in quite a few important ways and I wish it had pushed the characters more or maybe pushed the plot farther? I'm not sure. It just felt like a let down.