The performance is better in DX12 vs OpenGL, but the truth is pretty much DX12 is where the industry focus is and will be for the foreseeable future.
Stability, I believe is derived from the fact that there is constant (and consistent) development for DX12, bugs get reported faster, more often and thus give you stability.
The secret (once you're in the club) words they used in the release notes were 'modern development environment'.
Moving to DX12 is a good long-term plan. The fact we get HDR support now, and a small 'speed bump' is good, but long term is why this change is important. I suspect that we will see some features within premier get updates and performance will climb as portions of the code continue to get moderinized and stay in sync with the video card hardware and drivers much better going forward.
There is also a reduction in the cost of devolopement that Adobe will benifit from, but we will get updates and bug fixes quicker as the API's they need to manage gets reduced in complexity.
I also egerly await other persepectives, I look forward to seeing additional responses to your question.