I can't help with the Herpstat 6 as I've never had one, but a fast and cheap safeguard to prevent a UTH from overheating if your thermostat fails is to put a dimmer switch between the thermostat and the heating appliance. The dimmer should be set so that the appliance will go to 95-98*F, hot enough to be uncomfortable for your snake so you notice a change in behavior, but not so hot that your snake will get burned.

Then you set the thermostat so that your heat tape is around 90*F.