Part of the problem with the Spread is it requires an extraordinary athlete at the QB position. A true Spread-QB needs to be part Barry Sanders or Jim Brown and part Joe Montana.
The problem is there are not too many of those supermen type players around.
Whereas in a more traditional pro-style system, a QB with a gifted arm is almost as critical, there are many, many more players with the skill set necessary for success in that system. And even if you don't have a Joe Montana or Joe Namath or Troy Aikman on your roster, you can overcome that with a strong O-line and a solid RB. But, if you do not have a Tim Tebow or Pat White or Steve Young (who would in my opinion have been the PERFECT QB for the Spread) you are in big, big trouble.
The teams that can find the prototypical Spread QB from the very thin pool of prospects will succeed in this system at the highest level. Those that cannot or do not will not.
The UF spread, with all the speed in Harvey, Rainey,and Demps will fall on its face if Tebow is injured (like mid-season last year). Not that the back-up QBs are not talented, just that they are lacking in one or the other skill set required to make that system dangerous (one is a good runner but throws like Kodi Burns, the other a fantastic passer but runs like Chris Todd). If we, on the other hand, lose Wilson, we will suffer but we can still use our complimentary players at a higher level than UF could with a loss of Tebow.
This, in my opinion, is the real risk of the Spread. But, when the pieces are all in place, it is sure a fun thing to watch. Unless it is your opponent running it.