It's not that it will pry out, but that it will get between the edge of the plastic insert and the outer housing. There have been similar threads on that type of container and they were not recommended, but hey it's your choice, good luck with using it. I personally wouldn't use it.

As far as the UTH melting the plastic, that won't happen if you use a good thermostat to regulate the temperatures. See the BP caresheet for the correct temps.