I know that Manny Steward's empire, The Kronk gym in Detroit, is now part of classic boxing lore, and that a ridiculous amount of talent passed through those doors. While he obviously had a knack of crafting and honing a specific formulaic style that led a few special talents to the high ground of world championships, he did seem to burn out at least a couple of them very early (Kenty, Paul), presumably through weight-shedding. He didn't hold Hearns back in that regard, though Hearns probably had to be handled differently because he was such an otherworldly talent and a bit of a physical freak who could handle multiple weight jumps. McCrory was what he was and that was okay, but he didn't have the resiliency of Hearns. Am I being too critical there? Is there room for the argument that Steward had a few critical flaws in his developmental technique for his fighters?
I think he did a tremendous job when you consider the number of contenders and champions he was able to produce from just one city. Detroit rivaled traditional Boxing powers like New York City, Philadelphia and LA in prducing quality fighters during Kronk's heyday.
One of the things I read about in George Kimball's book "Four Kings" is that Hearns had such incredible sparring in fighters such as McCrory and Breland in 1984 that when it came time for his fight with Roberto Duran it was like Duran was in slow motion. Hell of an era.