I have a presto pot and stirring system with PID temp controller and when I need to work a larger volume I use it. That said, I know plastic guys have used hot burners, stoves, hot pots, etc. to heat plastic and you must stir or it will burn. I do use a heat plate to keep small batches hot while I’m working so I can do multiple shots without reheating in micro. Heat plate on high and set your Pyrex or Anchor right on it. Never had any troubles.
Personally, I haven’t use a hot burner to heat but wouldn’t be afraid to if that’s what I had. Start with a gallon first. Use safety precautions(ventilation) and a thermometer.