Nixos makes managing systemd services and timers a breeze. I use them for everything. Moving jobs between different machines is just an copy paste job since everything is captured in the nix config.
I love systemd timers, only thing they lack is error Mail Integration like cron. Which belongs one level up probably, but still…
You can use ExecStopPost= or create template service that will send e-mail on service failure.
You could use https://github.com/kbslabs/open-exec-wrapper (or similar - there’s a few options) as a wrapper which supports more fancy stuff like sending the script error to rollbar.
I guess the proper way of dealing with this would be to do some kind of integration via DBus to know if a unit failed.