at which point you have to go to specific distros or go through the PITA process of making it work, when AMD generally just works.

Ok, I agree with this point.

My counterargument is that those “specific distros” make up the vast majority of desktop Linux use. So it’s less that you have to choose a specific distro and more that you have to avoid niche distros.
Doesn’t invalidate the core of your argument though.