I think a huge problem is that the characters do stupid things that realistically they wouldn’t do. Particularly in the Canto Bight arc.
“Yup, them’s the shuttle parkers.” Seriously? The heroes are on a top-secret covert mission where remaining under cover is absolutely critical, and the first thing they do is ignore the instructions of the casino valet and rush off, leaving a cloud of suspicion? Are these our heroes or incompetent bunglers?
“Did you find the codebreaker?” “We found a codebreaker!” Because they’re all interchangeable, and it doesn’t matter if the heroes specifically failed to get the one guy Maz Kanata told them to, right? It’s not like the survival of the Resistance depends on it or anything.
Obviously this ties into the humor issue. I’d say a major flaw in the film is how with Poe, Finn & Rose it frequently goes for humor over characterization, to the point of undermining the entire idea that these characters are military operatives on what they think is a vitally important secret mission.