The melting point for most plastics are pretty high. If you're heat tape got up to the temps needed to melt plastic I would worry about the health of your snake.

Most important thing is to have a thermostat controling the output to whatever heat element you are using.

If your thermostat fails, which is possible with even high quality ones, then you're up the creak no matter what UTH you're using.

For one snake it might not be worth having a back up thermostat but when your collection starts to grow most people will back up their proportional thermostats with a ranco or johnson on/off type thermostats. In the even that one thermostat fails the other will ensure that temps don't get into a dangerously high levels.