Sorry if it sounded rude, it wasn’t meant to. I ran into several issues as well when using cron, but to make the point clear it was always an issue with how I used it. So the cron system itself was always working 100% reliable as it is meant to. I can only say that for Debian derivatives and their particular implementation but since cron is the most widely used automation system (even more than systemd timers for now I believe) I doubt that doubts about cron itself are helpful or the right direction in that other users should be pointed to. Nothing else is used thus proven reliable that much on that many different Linux derivatives.
If you faced issues I believe they are related to usage and scripts or the underlying OS setup (init system, service states etc). Nextcloud docs give clear steps about how to implement their cron job via crontab and if something fails it is most likely an issue with the Nextcloud instance (database, access etc) and/or PHP but not cron itself. It was in fact like this during my long Nextcloud experience and all user reports about failing cron I remember.
But enough about this now, I think my point is clear and I’m happy to helped verifying working Nextcloud background jobs .