Only reason you dont see any major benefits on Nvidia side on DX12 is that they actually have coded the drivers well from DX11 compared to the red team. I gain 0 FPS from DX11 vs DX12 in Rise of the Tomb Raider. It only benefits low-end systems aswell. Maybe thats why I dont see any difference either.
You are right. AMD's DX11 is abysmal.