Two things:
- steroids
- teaching to a system
Saban is an elite teacher of his defense. His kids learn to play it really well. But they are reliant on him and his calls, and don't do well transitioning to new systems. For example, Mark Barron, who cannot play a deep half for his life.
The old Miami teams played a defense that depended less on moving parts and schemes, and more on fast flowing to the ball based on a simple set of rules. Players can then learn to identify offenses and via instincts, instead of focusing on the varied rules and movements that add up to make it all work.
Saban is a beast, but his system doesn't help kids in the NFL.