If someone made a regular unicycle with a motor where you have to keep the balance yourself, either by pedaling or by exactly the correct amount of power using a throttle or the like it would be a totally different story and I guess this thing would be a lot harder to ride than a regular unicycle.
@Justin_LE motorized a couple of unicycles (one of them geared!) and inspired at least one other person to build one. Here are a couple of threads.