It seems as of 20.20, installing amdgpu-pro on Arch/Manjaro would cause the system to not boot properly. It appears to be a known issue on ArchWiki (that suggested workarounds to use it temporarily for things like DaVinci Resolve). I’m using Cinnamon by the way.
After installing amdgpu-pro 20.40 (using the PKGBUILD, but changing 20.30 to 20.40), X server would segfault, restart, then segfault again, in a very fast and endless loop, that I can’t even type properly from another tty (as X would automatically switch the active tty whenever it restarts). I only managed to break that loop by struggling to switch to tty2 (quickly mashing CTRL-ALT-F2), type my username, password,
sudo systemctl start sshd (while the system is still on tty2) then log in to that system from another one and uninstall the amdgpu-pro packages. After the packages were uninstalled, X started successfully and I entered the desktop.
Not sure what caused amdgpu-pro to break on Arch-based Linux distros. It seems 19.30 is the last known good version on Arch/Manjaro, which booted
EDIT: Not entirely without issues for 19.30. Actually, I tried installing this on my laptop which is using XFCE and I ended up in a white screen after login (so I have to uninstall it). On Cinnamon, however, there’s a minor issue that the terminal app would lose transparency.