They suck quite a lot at genitals too. But what makes hands especially tricky is simply that they are pretty damn complex. Hands have 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.