A plastic knob still won't give enough protection to comply with the safety regulations, in most countries: knobs can and do fall off. It the potentiometer is fixed to an earthed, metal panel then that might all right, depending on the creepage and clearances, inside the potentiometer.
As far as your query to the U211B is concerned: it depends on the type of motor used. If it's a universal motor (brushed) then it will be fine. If it's an induction motor, then you might just about get away with it, for a grinder, requiring little torque, but it's not the correct way to do it. A variable speed inverter is the proper way to control the speed of an AC induction motor.