The key requirement is abrasion resistance, of course. Obviously, they need to be very durable. And you don't want them snapping, either. The last thing you want is a brittle material. Apparently, that was the trouble they
had in the past, making the bur tough enough so it didn't break. I think part of the problem was heat, as well. Drilling into a tooth at high speed, you obviously get a lot of heat build-up. You need a good degree of thermal stability.