According to a good friend of mine that was a quad gunner in VN the Dusters only draw back was the shells would detonate on impact. I can't think of the name of the fuze. So in the heavy jungle the shells would detonate before penetrating too deep. The quads fired 50's which would just tear into anything they shot at.
The guys who rode those Dusters had to have balls, when the poo hit the fan they had to get on top as you see in the video, and feed the guns. If you are interested there is an excellent book called Dusterman written by Joe Belardo, it's an amazing read, gives great insight into the courage these guys had.