Then, we disagree on this, I say within the general trend of actual events, that this is proof enough.
-Dozens of DX12 titles have broken implementations on both AMD and NVIDIA comapred to DX11.
-Actual developers (their testimony is features in this thread) saying that DX12 is much harder to do than DX11, and the fps gains are harder to come by, and that DX12 should be considered only when you can leverage it's advantages (ray tracing, lower CPU overhead, exotic rendering ..etc).
-Several testimonials stating that some developers have problems circumventing "undefined API behaviors" due to mismatched descritors on NVIDIA GPUs.
-Several games have way slower DX12 implementation on NVIDIA hardware specifically, despite their DX11 version (or the previous games in the series) working fine with DX11, sometimes even the next game in the series work well when it had time to mature enough as in the case of Borderlands 3 vs Tiny Tina.
-On the other end of the specturm, dozens of DX12 games work as intended on both AMD and NVIDIA GPUs, like Gears, Forza Horizon, Control, Cyberpunk, Metro Exodus, .. etc, from developers with a good reputation of good software design.