The documentation reminds me a lot of Emacs's "ANTINEWS", listing all the functionality you lose by downgrading to the previous version of Emacs. Except in this case, it's actually a "NEWS" file, showing all the functionality you lose by downgrading from systemd to uselessd.
> Certain superfluous unit types removed, namely devices, timers, swaps, mounts and automounts.
So under uselessd a service or socket file cannot depend on the presence of certain mounts, for instance (RequiresMountsFor=). Nor can a service run maintenance tasks periodically with supervision on the maintenance task, and with all the privilege-dropping, isolation, and security features easily accomplished with a systemd service. (And even if a cron implementation implemented such features, they'd work gratuitously differently from systemd services.) Nor can the availability of a device trigger the reliable, monitored launch of a service with all those same systemd features.
> Setup routines for various MAC/ACL systems, including SMACK, IMA and SELinux, are gone. We want to stick to a more clearly defined purpose, one that is agnostic of such elements. Nonetheless, we have retained SELinux access routines in D-Bus APIs and unit options for SMACK attributes in socket unit files to respect existing configurations.
So anyone who actually wants to use such security systems is screwed, then. Good thing Linux isn't about choice.
> systemd-fsck has been replaced with a service file that starts /sbin/fsck to fsck devices. In essence, this isn’t much different from what systemd-fsck already did, but with the overhead of a middle man executable interfacing with /sbin/fsck cut out. This also means that the systemd-defined sysctl parameters for its fsck are gone, and you should use the old ones (/forcefsck, etc.) instead.
So now, in order to force a check of a filesystem on reboot, you must write to that filesystem, potentially doing further damage to it.