Any quarterback is a gamble. Leaf and Manning were considered toss ups in talent. Tony Romo was an undrafted free agent. Tom Brady was a sixth rounder.
The rest of the “risky” picks are guys that have questions about maturity or guys that have a lot of raw talent but haven’t developed skills.
Some guys grow up, but maturity is always a concern with guys who are 23 years old and suddenly have several million dollars.
Guys with incredible raw athleticism sometimes rely on that athleticism and don’t develop their skills. That can be good or bad. DeMarcus Ware was working on developing a ton of different moves and his position coach finally told him, “You’re faster and stronger than most of the guys you’re going against. Develop a few good moves and quit negating your talent by outsmarting yourself.” That said, the top athletes in college only go against people as good as they are maybe twice a season. In the pros, it’s far more rare for a player to be able to simply “out-athlete” the other side.
Hey, don't forget Kurt Warner, undrafted yet has two NFL MVP awards, a Superbowl MVP award, and a Superbowl ring. A guy who was undrafted and rattled around in the Arena football league for 4 years before being given a shot!