As I understand, a problem with the theremin is also that the sensitivity depends on humidity and anything else that affects capacitance, and capacitance also depends of the shape of the hand (and arm position, and body position, etc) and because of that it doesn't directly translate to linear distance coordinates. So it becomes hard to build reliable muscle memory. The leap in principle should be able to avoid these problems, although so far a lot of the software written for it fails at this.

Also, a theremin is always on, you can't for example turn the detection on or off based on the amount of fingers you hold up.