AMD has just historically been better, while NVIDIA terrible. The fact that NVIDIA became better is a relatively recent event, after they decided to start putting out official Linux drivers, and even then it took a while for them to make that stable, afaik.

Meanwhile, and this is also afaik, AMD has basically not really changed by much. NVIDIA just leaped them on the compat front on newer cards.

I only switched to an AMD GPU this year after 5 years using Nvidia cards. I assure you my situation is more common than yours. Your experience is also why people keep saying "Nvidia is OK now", meaning quite clearly that it was absolutely not great before.

With Nvidia, once you got it installed and stable (if you were lucky your distro made that happen), it might stay good for many months to even a year. Then BLAM, some update catastrophically breaks your system and you're stuck booting from a thumb drive trying to un-fuck your system. This was especially rough for me as I used my main system for work and gaming, so it had my Nvidia GPU in it.

My system has been smooth sailing since I switched to AMD. The drivers are in the kernel so it should be about as easy as it gets and shouldn't be as finicky about which distro or DE you use. Now keep in mind that I am also on a top 7000 card, not a 9000 card, so I can't speak to them from experience, but I haven't heard any horror stories.

That tracks with what I have seen with my own cards. I suspect some of it is because there were specific driver versions that many distros favored for stability reasons that may have left some performance on the table, but that's just my personal speculation.