Domestic hot water shouldn't usually go over 140F. The pex is good for over 250F. It has been used and tested in Europe without a problem for 20 years. It is the same material as is used in the majority of radiant heating systems; although it doesn't have the added oxygen barrier. The companies who make it are being very careful not to allow problems. In many ways it is tougher than PVC. PVC is used regularly, but it is limited to 150F. People were stung by using another flexible plastic plumbing product; actually it was the securing rings at the fittings that were most of the problem.