We did have ours freeze up once (-17 in 2003, I believe), but no internal damage to the unit or pipes, for that matter. I didn’t install the little thermostatically-controlled electric resistance heater for the unit so I was lucky. Since then I improved the insulation and installed a little fan to blow warm air from the adjoining room on it when needed, but it has never been nearly that cold again.
OH, I do have heat tape a little upstream too which is probably the main improvement.