FWIW, I've used the Web USB installer probably a dozen times without issue, so I would definitely recommend that route if adb is intimidating. I've done it both ways, but the web installer is so easy it feels magical.
As for it not working, were you running a Flatpak/Snap version of the browser? Apparently those are problematic with Web USB. I personally haven't tried on Linux yet (just Mac O's) but now that I've switched to Fedora I might encounter the same issue you did.
Tip: The GrapheneOS Web installer doesn't work for me with the Chromium in Debian Stable.
I always try the Web installer, to see whether it works now, but then end up doing the command line method (which is also pretty easy, compared to historically how Android phone flashing used to work).
As for it not working, were you running a Flatpak/Snap version of the browser? Apparently those are problematic with Web USB. I personally haven't tried on Linux yet (just Mac O's) but now that I've switched to Fedora I might encounter the same issue you did.