Tebow is not an awful QB, but he's an awful NFL QB - as dictated by the current schemes, expectations, and dynamics of an NFL team. When people say Tebow is awful, that is essentially what they are saying.
Edit: And going only by that he is a "winner" to judge a QB - regardless of how he performs 55/60 minutes and in practice, he asking people to go on blind faith that he'll win you games. Well, you're not going to find a whole lot of people willing to base their opinion, nor make a business decision based on blind faith. And when I say faith, i don't mean religiously.