It's the nature of sports; the 'better team' doesn't always win, and frankly without this phenomena, almost nobody would be interested in watching sporting events.
Why do you completely disregard the fact that at times BYU won because Bronco did an exceptional job prepping his team and outcoached superior teams? Doesn't fit with your narrative?
By any subjective measure, Bronco is a good coach. Add the unique BYU factor, and I think he's in the great category. Does he do things that are frustrating at times? Yes, but so does almost every sports coach the world over.