The "can only be used in /etc/fstab" systemd settings are essentially workarounds to do those things via fstab (and workaround fstab related issues) rather than depend on other systemd facilities (c.f. systemd-gpt-auto-generator). From a "what can you do in /etc/fstab without knowing systemd is working behind the scenes" point of view, then yes, systemd units are vastly more configurable.
Are you surprised that such a service exists? I certainly was. And doubly so because it has unusual dependency requirements that can easily lead to deadlocks. And yes, this is known, there are open issues, and they are ignored.
> From a "what can you do in /etc/fstab without knowing systemd is working behind the scenes" point of view, then yes, systemd units are vastly more configurable.
No, they are not. In my case, I had to use fstab to be able to specify a retry policy for mount units (SMB shares) because it's intentionally not exposed.
And yes, there's a bug: https://github.com/systemd/systemd/issues/4468 with the expected GTFO resolution: https://github.com/systemd/systemd/issues/4468#issuecomment-...
So there's literally functionality that has been requested by people and it's available only through fstab.