I was so close to buying a year of Uptime Roboto but I decided to have a search on Appsumo - then I found Pulsetic.

The UI is great and simple to use, and there are heaps of integrations. There is no app like Uptime Robot, however, you can integrate with SIGNL4 - which is a critical alert app and will work the same. You can set alerts on the account level or the watched domain level.

My feedback for the team is this:
Allow tests with the SIGNL4 app.
Look at an integration with MainWP - similar to Uptime Robot