I thought he said he started out in sanitation?
And someone can do lots of training simulations, pass with flying colors, and still choke when confronted with the real deal. Remember Lt. Gorman in Aliens?
They weren’t on some top secret government mission in Aliens. I could totally buy a random stormtrooper deserting in most settings. I’m sure they have some tangible desertion rate. It defies all credulity though that someone like Finn would be involved with Kylo’s mission on Jakku.
Then again he did manage to stand his ground against Kylo Ren in a lightsaber duel so clearly he’s not without combat skills. His courage also seems to come and go a lot in the film. I think his defection was meant to be more of a moral and ideological one. I can’t remember if it’s in the actual film, but the trailers put emphasis on him having lost the will to fight for the First Order. He’s essentially just fed up with it. Also his taunting of Phasma at Starkiller Base kind of suggest that he’s tired of being order about as well. But all in all the film is partially about him finding something worth fighting for.
There really is far to much information to process in this film. I really feel like a lot of important stuff was cut out. I wish we could have seen the 4 hour version they talked a few months ago.