And you immediately lose the ability to do `crontab -l` on any server to know its scheduled tasks.
Now you get to look around the myriad of places where you can put systemd files, and figure out which ones are base services and which ones are custom, with no general convention to go about it. Nope.
Bad memories. I particularly enjoyed fighting with third-party programs that installed system cronjobs in the various tabs, and having to remember to go and find them after package upgrades and try to figure out how to robustly identify when their processes were running so my other cronjobs wouldn't overload or clobber state, since the third-party-installed jobs didn't play along with any lockfile-based coordination we used. Wants/WantedBy/Requires are godsends by comparison.
Now you get to look around the myriad of places where you can put systemd files, and figure out which ones are base services and which ones are custom, with no general convention to go about it. Nope.