I agree that the lack of game experience is the single biggest risk factor. However, in all of your examples of QBs who lacked game experience that failed, every one of them was rushed or pushed into starting immediately. Also, most of them on your list weren't known as mental maestros coming out of college.
Here's what I'll give you: this is a unique situation, the likes of which we've never seen before.
But because his greatest trait is his football IQ and because he'll be developing under Stafford and McVay for a year or two, I believe the lack of game experience risk is greatly mitigated. Time will tell.