GaN HEMTs are very reliable, but the precise answer is still being refined. Reliability relates to anticipated useful life and is typically measured in mean-time-to-failure. Reliability can be anticipated and determined in advance by testing and measurement. Proving and improving device reliability is an ongoing process. It begins with a device under test (DUT) being stressed until it fails, that’s followed by identifying the failure mechanism(s), developing cause and effect models, and improving subsequent device designs or fabrication processes, hopefully with higher reliability.

Source : How reliable are GaN HEMTs? – Power Electronic Tips