I think the problem is the dull characters. Each has an interesting backstory that should make for an interesting character but watching them interact they all looked so bored. Cassian says he’s been in this fight since he was 6 but we don’t learn why. What motivates him to push further and further towards grey morality, in essence becoming as bad as the Empire to fight the Empire? That should be interesting but it’s never explored.
Maybe this is why I don’t get all the hate for Rogue One… I feel this way about every movie. Movies are too short, I find, to really get attached to the characters or explore them fully. Obviously, some movies do this better than others, but I always need to spend more than 2 hours with characters to really feel an emotional bond. Even still, I felt just as connected to the Rogue One characters as I did to characters from other good movies, so I feel like it worked well for what it was trying to do.