DX12 stability will depend on how many GPU/driver/OS combinations they test, how motivated and capable they are to fix problems, and their relationships with GPU vendors.
DX12 is extremely low-level and demanding of those who use it. It means the application developer has to take on most of the work normally left to GPU drivers. This includes things like low level details of supporting different generations of GPUs from the same vendor. The claim that it will "work absolutely fine with all DX12-compatible GPUs" is a bold one.