Some more context would be nice... Is that a clean install or an upgrade? Did 1.3.2 work?

Prior to the 1.3.4 update I had been using the retroarch 1.3.2 package found in the AUR (https://aur.archlinux.org/packages/retroarch), which worked fine. With this latest update the package apparently got added to the official repos and the update got pulled from there, but ever since that upgrade it won't launch. I ran pacman -Rncs retroarch and reinstalled it, but I get the same results.

It's probably just a configuration issue, did you try deleting your configuration? Both 1.3.2 and 1.3.4 work just fine here.

I just ran pacman -Rncs retroarch again, and then deleted the RetroArch configuration file at /etc/retroarch.cfg. Reinstalling after that did not change anything, it crashes with the same error as above. I then tried installing the retroarch-assets-xmb package just in case that was causing the issue, but that changed nothing as well.

I'm using Mesa with radeonsi if that helps at all.

Dude, that file is a default config that comes with the package, it is required and should work at all times. The configuration you're looking for is located, like virtually everything out there, in ~/.config.

I also had the same problem, it would just crash on launch. running reatroarch --verbose told problems about config, at which point (like mentionned above) renaming the folder .config/retroarch enabled it to launch.

Of course the new config tht gets created is a clean slate, but you can still re-add files if needed (or reconfig, it's rather easy), but at least it does fix the crashing.

I was only aware of the config mentioned on the wiki, deleting the old config did work. Thank you.

Everytime I refreshed the page it posted another comment? Now I'm really looking like an idiot.

You can delete comments, it's no big deal. Anyway, closing this now. Keep an eye out on changes in /etc/retroarch.cfg via pacdiff, this'll usually tell you when the default config changes and that you need to change yours accordingly.