You’re right, I don’t have mint/ubuntu installed nor that kind of hardware (anymore), so I can’t give precise instructions. I was just like: see that you’re not missing any of these packages/repos/firmware and adapt it to your needs. I had to deal with a laptop with dual gpu (intel+amd) and it was such a pain in the ass to get it working. I think you needed to have n packages installed, add grub flags, configure X11 to use amdgpu and blacklist radeon and even when I had it working, the amd gpu was only compatible with a limited amount of vulkan instructions so I had graphical glitches and games breaking. Old dual gpu setups are just a nightmare.
I had that warning too with a r9 270x and if I remember correctly it was because radv didn’t have official/full support for GCN =<2, so what they’re saying is something in the line of… it may break. In my case, it worked beautifully gaming with proton, etc.