These things are PERFECT for agitating an etchant bath for folks who make their own PCBs.
Was surprised to see any kind of feedback loop at all. I've had to repair a few lab stirrers and whatnot over the years, and most of them ran straight open loop phase controllers, similar to an incandescent lamp dimmer. The really simple ones simply used a large (25W or so) wire wound rheostat in series with the motor.