The problem with prospects is often times we evaluate them and talk about them in the future of being boom or busts. Every guy that hits well and pitches well in the minors is a future MVP/CY young guy, that may be a bit hyperbolic but I think it makes my point. Johnson always projected as a back-of-the-rotation starter. Often prospects don't reach their ceiling and they become something in-between. For some perspective, let's look at a guy like Houk, who many envision as this great starter who is going to come up and take a spot in our rotation. Could he? yes, but I don't think he's any more than a back-of-the-rotation starter if he can refine his offerings. You can get guys out in the minors with two good pitches but you'll get chewed up in the bigs the second time through an order. So what happens is everyone scouts the box scores and assumes this guy can transition his MiLB dominance into the majors but that will never be the case if he can't refine his stuff.
Will Houk be a bust? maybe, maybe not if makes some improvements, but he could also fall somewhere in between and become a reliever, a very good reliever at that too. Think Matt Barnes. Matt Barnes failed as a starter and now he's one of the best closers in the game.
I think of someone like Duran too, who I think could have a really good MLB career, but people talk about him in here as if he's going to be an all star who will immediately make this team better. Maybe he will help this team right now (can't do much worse than Santana) but there's a good chance he's not going to be as good as many think he is.