Boyega can likely do more than what he was given, but that's what he was given.
- It makes no sense how he was indoctrinated for practically his whole life, but suddenly has a change in conscious. Sure, he's never killed people before, but you'd think that preparing him to kill ALL those that are against them would've included videos of them killing innocents, and of the Deathstar blowing up planets. The dude had NO signs of disobedience, nor did his mental records show that he had any inkling of resisting the indoctrination. It makes no sense.
- Why is he so freaking enthusiastic, and informal? He's been indoctrinated, and trained, in a cold, harsh, and strict military environment for nearly his entire life. You'd think he would be more formal, very awkward with informal dialogue, and uncomfortable now that he was free of the First Order, because he's never lived without regulations, and rules.
- He describes the Stormtroopers as indoctrinated victims since either birth, or from toddler age, YET he has NO issue with casually killing them. In fact, he does so gleefully; yelling "WOOO!!" while doing so. I know he has to kill to survive, but he should feel bad because, not only are they victims of brainwashing, but he's grown up with them practically his entire life. There should be grief, and grimacing on his face, and then at some point he just forces himself to detach himself from the situation and do what needs to be done, but in his private moments he's seen with his head down.
- He's a coward and we know he was a janitor...so why not tie those two things together? How about, even tho he never showed any signs of dissent during his training, he always had a shyness towards violence which affected his performance, which is why he was given sanitation duty for a period of time. However his marksmen skills were undeniable, so he was brought back to complete his training, and eventually deployed on his first mission years later. Easy.
Fin's character is just a generic character with some unique aspects that were never built on.