My frustration is that systemd isn't working in this case, that it is trying to do something pointless (check an already mounted filesystem), AND that I have no way to diagnose or correct it since there is no useful information to work with.

When I had my drives on different controllers things were even worse because btrfs device scan didn't complete running before systemd then tried to mount an incomplete filesystem.

Automatic partition discovery is going to have its own issues. For example plugging in a drive from another system/backups is not something you want mounted in the right places on the wrong system! fstab is also a nice place to put non-physical filesystems like tmpfs (I use it for /tmp /var/tmp $HOME/.cache $HOME/.ccache /space/work). Having to create separate .mount files for each of those would be a big pain.

I really want to use and like systemd, but it does fundamentally need to work first.