Lots of pots designed for mains isolation, that's how all TVs used to be. There is nothing wrong with designing a drive like this, as long as the speed control is manual with an isolated pot. My KB drive is industrial, they have a whole range of them.

Miller, important to note that the circuit in the Mach forum has to have its own mains power supply to maintain isolation. The trouble is that it needs a supply above 10 volts to get 10 v max output.