Actually it works fine on Steam Deck. It uses VRR over DP to the dock, which then translates it to HDMI with VRR. The dock has proprietary firmware to do this.

Intel and Nvidia hardware with open source kernel drivers also do a similar trick where the HDMI part is in a firmware blob. Only AMD does not work with HDMI VRR.

Now, if you want. There will probably always be tradeoffs between the two drivers so I doubt this will ever match Nvidia’s across the board, just have to pick your poisons.