The way I see it the transfers that were run off by coaches, faced a scheme change making them leave, or left for discipline reasons seem to have a better chance at doing well at another school than someone who transferred out because they weren't good enough to start at their original school. Chances are you aren't going to end up on a more talented program than before if you lost your starting job. Obviously the exception there is Coker went from stacked FSU to stacked Alabama after getting bumped down by Winston. Maybe it's just memory bias but it seems most of the "successful" ones like Mayfield, Newton, Wilson, Aikman, etc fall into the former categories.
I think most transfers fall into the latter category which is why most people don't see it as a typically successful move. They probably aren't just going to get magically much better with just 1 offseason in a different program.