I can name a rather large problem with cron that systemd timers solve handily: long-running job duplication. When jobs take longer to run than the space between their triggering times, duplicates start piling up. I've had to rescue numerous systems from such states, which are difficult to detect until things have gotten quite bad. Sure, you can write a bunch of boilerplate to handle this yourself with cron, but with systemd timers it's all handled for you along with other niceties like capturing all output in journald and ensuring that the next run starts at soon as possible.
Yeah, tell me about it.
We had a production-down support ticket filed for one of the things that I work on at $DAYJOB. The customer's VM's disk was full. Why? Because the 'timer unit' that was supposed to run logrotate every day had never been run and was never scheduled to run. (The VM had been up for a month at this point.) No other customers had ever reported this issue, and we'd not changed anything about that timer unit or what it is supposed to run in ages.
No amount of gyration or agitation with 'systemctl' and friends OR rebooting of the VM kicked the cron replacement into proper functioning. 'logrotate' was simply never being scheduled to run. This fucker was WEDGED, and the tools weren't helping us understand why.
We read through all the docs on all the various kinds of units and don't see what we're doing wrong. We do a BUNCH of digging, and find an -IIRC- open Github issue from years back where someone was running into this problem. More or less the last word on the issue was Poettering saying something like "Well... actually, now that we've said that that particular cascade of options isn't actually supposed to work, now I'm not so sure that they're NOT supposed to work.". And that was -apparently- that.
IMO, when your cron replacement can be easily configured in such a way as to never even try to run the thing that you scheduled it to run, you really need to go back and rethink how you've built your cron replacement.
Was not aware that vixie cron was first released 1987(https://github.com/vixie/cron/blob/master/Documentation/Chan... 1.md), and still has fresh commits.