For one, we don't know what how good they actually are/will benjoy throwing the ball. RG3 and Kaep have 2 yrs of data to go from (1.5 years in the case of Kaep).
Vick, Cunningham, etc. all had variable amounts success throwing the ball over their careers. Vick was up for MVP, what 3-4 yrs ago?
And regardless of any of that, the reason why these guys are comparable is that they run a very high amount and got injured frequently in their careers.
McNair, McNabb, Vick, even Young all missed significant time due to injuries in their career, more so than their peers at the time. I can't think of many mobile QBs who were good, that missed time quite as frequently. May be Schaub, Warner (who was older when he came into the league), Bledsoe, and Trent Green are the most recent examples I can think of who were considered good QBs that missed significant time to injury. Even Brady and Peyton who missed 1 full season to injury, didn't miss any before or since then.