I'd say you have that exactly backwards. It's velocity that eventually dooms all ballplayers, whether that be when they step up from coach pitch to little league, from college to pro, or as a MLB player's career winds down. It's not that hard to hit breaking stuff, once you learn to recognize it. Having the hand-eye coordination to handle velocity isn't something that can just be learned, you either are born with it, or you drop out of baseball at some point. He has handled breaking pitches just fine in Korea, we'll see if he can consistently catch up to the additional velocity he's going to see in the big leagues, without having to cheat. To me, it looks like he's going to be just fine, but if he fails, it'll be velocity, not breaking balls that dooms him, IMO.