Using Variacs with Low voltage lighting

I have read lots of posts regarding variacs for hum free lighting. Is there a problem running "off the shelf low voltage track lighting" with a variac? I seem to remember something about the transformers and variable voltage. Also I have seen low voltage lighting that runs off one main transformer w/ the dimmer on the low side of the transformer, has anyone had any experience with these

Low voltage light have specific dimmers. There are electronic and magnetic. Lurton has a excellent selection of both types.