Brown must be a great coach because everybody says so. After all he coached from age 32-70. Unfortunately his career stats are .476 for the ABA and .518 for the NBA.
If you look at his career, he's only good for about 3 years at any one stop. I bet he's coached more professional teams than any other coach. Successful coaches usually manage to keep their jobs and build on success. Doesn't happen with Brown. He is a perfectionist, asking humans to be perfect and eventually he gets crosswise with his players and is either fired or quits.
I'd say that the aspect of coaching that deals with measuring your players' skills and temperments and then attempting to build a team are simply beyond his ability because of his own psychology.
But he must be a great coach cause everybody says so.