Ok well, the Hub is just incredibly useless. Doesn’t give any information IF the installation worked/failed, nor does it tell you why it’s failing. After using Unity for 9 years it’s really sad to see that Linux is still so badly supported - even UI scaling isn’t available (but it is on windows)…
You can find the actual logs with USEFUL information here:
~> cat ~/.config/UnityHub/logs/info-log.json
Turns out cpio is required (and of course there isn’t any warning/notice about that), so you can just yay -S cpio that and it works afterwards.