Well, to be the devil's advocate,sometimes the fact that grappling on the ground you get stomped or you get injured by broken glass, needles, rock and so forth is an excuse for not studying groundwork. Being realistic, with the adrenaline dump kicking in, the type of surface you are rolling in becomes secondary. My main concern with BJJ is that sometimes has an excessive sport component that compromises self defense attitude. But this is not for BJJ but for the many others MA ( Karate, Judo, Olympic Tae Known Do and so forth).