Feels.
Even the pro scouts can't predict this. He has the size, arm, and quickness to be a prototypical professional QB. His major issue is that he played for Harbaugh and they were so good relative to their conference and nonconference schedule for much of his career he didn't get to put up gaudy numbers. The other issue is health.
Tebow was a hype-created monster who threw a football like a baseball player, didn't have a strong arm, and wasn't a good decision-maker. If you want to compare McCarthy to someone there are about half a dozen 2nd string QBs in the league that he resembles.