The flaws are primarily a problem of the two main characters, and begin in TFA:
I would say Finn’s defining trauma is that of being a Stormtrooper realizing in his first battle that he’s not a killer. However, he immediately starts killing his own mates when the story needs him to, and this aspect of his character is never explored again.
I have seen a few people comment exactly the same type of thing, but they have misinterpreted that scene, even when it’s explained later in the film:
But my first battle, I made a choice. I wasn’t going to kill for them. So I ran.
So it’s NOT that he isn’t a killer, he just refused to kill innocent civilians and that is the moment he sees them for what they are and decides to run. Slaughtering innocent unarmed people and killing during battle are two completely different things.