It's strange how things are so easy to do but need a so complex procedure by our brain.
Moving the mouse is something natural for most regular computer users, but actually need some complex "feedback driven" structure in our brain.
Just think when you move the mouse the feedback chain is more or less this:
This all works fine for most of us (well everyone some time lose the current cursor position ;) ), there are some people that have some major difficulties with the mouse usage, many of them just need some pratice but some of them, doesn't matter how much do they pratice, just cannot take control. I think that it's due to some "lower precision" of thery control of the wrist and/or arm, in some of these case I've found that just use a different device, like a touch pad, sole all their problem.
As always any computer interface if just like a pair of shoes, you have to buy those who fit you the better ;)