From my own experience (which granted isn't nearly as extensive as Hall's), I've found the same things to be true.
It always struck me as basic common sense that you want to be on top, yet tons of players are comfortable being off their back. Way too comfortable. I think this has a lot to do with the BJJ mentality "smaller man beating bigger man with technique". Sure, you can play on bottom against someone who's not as skilled as you, but if you're equally skilled, he'll have a mechanical advantage, then you'll either have to rely on tricks or surprise, neither of which are really reliable. The problem is there rarely is equal skill levels, and even at the highest level people can compensate with unorthodox technique or tricky flexibility.
The other thing is how sweeps are taught in isolation. As exposure it might work, but to actually explain the technique in terms of "leg goes here and push" is completely ignoring the basis of what makes a sweep work; off-balancing to get positional advantage.
