i dont think duff is techincally that good, as much as i hate to admit it
he definetely does his job but thats about it, i think it was joe perry that said he was like the bassist from ac/dc, very simple but got the job done or something like that
in bands such as GN'R and VR, that is the definition of being a great bassist! steady as a tank.
guys like Flea and Les Claypool is probably more technically skilled, but they would never ever fit in those two bands!
Duff has a great feel and makes very interesting basslines too, but slapping and popping isn't his style, just like tapping isn't Slash's style.