God it's time someone came up with a good, modern monitoring system. I used Nagios for years but it never evolved past a bunch of CGI scripts written in C(!). I tried Sensu, and was moderately impressed until a major update broke everything and it never worked again.
I've used Nagios in quite big installations. There are plenty of things that can be improved, and has been in the surrounding ecosystem, but that's just paiting a false picture. Nobody runs CGI anymore, and even if they did, they're not in C and never has been. Some of the checks are, of course.
