There are many fun reasons to have a character avoid using guns, but I think "this otherwise-awesome fighter is a really terrible shot" is an underappreciated one. Yes, they're available in my setting and would work against my enemies. No, I don't have a moral code against them or a power that makes them redundant. I just can't aim for shit!
In general IMO the more badass and hypercompetent a character is, the more they're enriched by having one skill they're just absolutely hopeless at. Terrible shot, terrible liar, terrible dancer, no poker face, no sense of direction, can't drive, can't swim, can't cook, can't read, always broke, always late, always falls for a certain trick...
Commander Shepard is an imperative example.
