The RGB control is a kernel problem not an OpenRGB problem (well, it might also be an OpenRGB problem if the card doesn’t work in Windows either). The amdgpu kernel driver doesn’t expose the i2c interfaces not associated with display connectors, so the i2c interface used for RGB is inaccessible and thus we can’t control RGB on Linux. AMD’s ADL on Windows exposes it just fine.
That said, I can’t agree that NVIDIA just works. Their drivers are garbage to get installed and keep updated, especially when new kernels come out. Not to mention the terrible Wayland support and lack of Wayland VRR capability. I’m happy with my Arc A770 (whose RGB is controlled over USB and just works, but requires a motherboard header).