I hated that they killed of Ilsa.
It was nice to finally have a female agent (or even character, Julia aside) show up in more than one movie. And for this franchise, she was well written, a skilled woman was also flawed. I liked the relationship between her and Ethan and how it progressed between Rogue Nation and Fallout, and was actually looking forward to a potential romance between them.
Honestly, if they had to kill someone off, they should have killed off Luther. I like the character, but him being there from the first movie would have given killing off one of the main characters the impact needed for raising the stakes for Ethan while keeping Ilsa alive. I get the actress wasn't available to film as much as they like, but they could at least have had her doing side missions for Ethan that tied into the larger narrative, without having to kill her.
Other it was a decent if convoluted movie. My favorites in the series are still the first one, Rogue Nation, and Fallout. I'd put in the mid rank, along with part 2 and Ghost Protocol. I hated the third one.