As it has gotten closer to draft time, a report came out that states two NFL scouts claim that Griffin is selfish, but that is the first report of that type I know of and of course he has rejected the claims. However, it may come out in the bigger lights of the NFL if in fact that is an actual characteristic of his. The only real worry I see on the surface about RG3 is that he played in a spread system in college rather that a pro style form. There is no doubt he can throw the ball, but playing under the center in a completely different system may take time to become acclimated.
Some of Andrew Luck's only noticeable bust factors are the possible expectations by some that he will be the next Peyton Manning and that he is coming off a college career where he faced very little scrutiny which we know can be much different in the NFL. Maybe everything is too good about Luck, but if there is anything, it would come out after a couple bad seasons.
Also worth noting, injuries are a possibility for any NFL player and bad injuries early in their career could lead to bust not necessarily at fault of the player. I would guess that most people think that Griffin has the higher chance of bust, but Luck will be under the most pressure. Debate has already started as to who will have a better season and they may always be linked whether it be through failure or success in the eyes of spectators (think Peyton Manning and Ryan Leaf).