I'm not guaranteeing he's to be any good, in fact I would prefer they draft somebody else. But if they decide to draft him, I'll trust their that their evaluation is more thorough than your's or mine could possibly be.
To completely write him off at this point of his career is simply being shortsighted. He's a unique talent, you can't teach that arm, and he sounds like a smart kid and loves the game... but that's only from what I've seen. GMs/scouts have talked to all his coaches, college, high school, junior college, teammates, trainers, etc... they've turned over every stone... stones that we can't get to. So if they decide he's their guy, then I'll trust they've done their homework.
And if does become a bust in a few years, then so be it, but there's no guarantee Rosen pans out either... what if suffers a handful of concussions in his first couple seasons? Will his shoulder hold up? Will his teammates accept him in the lockerroom? Who knows.