Originally i self hosted it, but their free plan is more than generous enough for my needs (20 jobs monitored for free i think).
Besides monitoring backups and cron jobs, i use it for monitoring the power state of my summerhouse. We had a problem with a breaker that would trip for no reason, and since nobody wants to arrive at a summerhouse where the fridge has been unpowered for a couple of weeks, i put a RPi Zero W up there on the circuit of the problematic breaker, and set it to call Healthchecks every 15 minutes. If it misses 2, i (might) have a problem.
So far the alarm has gone off twice, both times (fortunately) internet connection releated.
`curl --header "X-Api-Key: your-api-key" https://healthchecks.io/api/v1/checks/<uuid>` does not contain the passed request body.
I've been using both for a while and they're great. Healtchecks has first class integration in borgmatic[3], and is in my opinion the better choice for cron jobs. Uptime-Kuma has a really nice interface and also supports checking hosts/services via ping or http request. It also supports "push" requests a la healthchecks but there's a weird race condition bug that makes it misfire (PR is waiting to be merged). It also has status dashboards with incidents.
[0] https://healthchecks.io/
[1] https://github.com/louislam/uptime-kuma
[3] https://torsion.org/borgmatic/