Some are and I have experience with that. I have no doubt my coach knows how to fight and he knows the sport but his idea of training outside of the boxing gym is running 10 miles a day, period. His mentality is that it worked 30 years ago so it must work now :redface:
I don't think speed bags are good for much. Even when I'm going fast at it the only thing that I feel is burning in my shoulders. I don't think it builds any power or endurance to a very high degree. I think its best attribute is probably rotator cuff conditioning.