What kind of feature set do you need out of it? Are you opposed to spinning up an open source project?
If you need something more custom like creating tickets in a bug tracker, it would be fairly trivial to trigger a script based off one of the notification options they have out of the box.
For a few years I've been running a couple self-hosted instances of Uptime Kuma from two different locations (a Raspberry Pi at my house and on my VPS) which monitor a few things I care about plus each other, and haven't had any problems.
In the last few years, they’ve made a ton of changes to their pricing and terms, and honestly, it’s been pretty frustrating for users.
We switched to Pulsetic monitoring a year ago, and when UptimeRobot first changed their pricing and terms, we made the move. Here’s the comparison table: https://pulsetic.com/uptimerobot-alternative/ . So far, everything has been great.