Spiral mixers are efficient and effective dough mixers. They differ from the planetary (or KitchenAid) style in two very important ways.
First, there is an agitator arm in the center of the bowl that prevents the dough "riding" up the hook, getting stuck, forcing you to stop the machine and slide it back down.
Second, and most importantly, they are specialized; designed with one purpose in mind, kneading dough. To achieve that, the bowl rotates around a rotating, and fixed, dough hook.