I've never been kicked full force/flush in the balls, nor have i experienced a full force and flush liver shot, but i've been told it feels about the same.
Others said that even with a cup, it's likely that we'd have regular injuries if groin kicks were allowed, so i am not in favor of it.
If that (the injuries) wasn't the case though, then i don't see one why couldn't think about implementing it. It'd just be another liver-like target and also make the sport more realistic.
@Ares Black I'm not sure if fighters couldn't learn to defend it in some way, though apparently it's too riskful regarding injuries anyways, so i'm not advocating for it, lol.