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.