They suck quite a lot at genitals too. But what makes hands especially tricky is simply that they are pretty damn complex. A hand has five fingers that can all move independently, the hand can rotate in all kinds of way and the individual parts of a hand can all occlude each other. There is a lot of stuff you have to get right to produce a good looking hand and it is especially difficult when you are just a simple 2D algorithm that has little idea of 3D structure or motion.