I do like Ubuntu it definitely is a good base distro for a MythTV system. It's by far the most popular OS according to our Smolt stats over 80% of MythTV users are using it! There are semi official PPA's available for both fixes and current master and plenty of helpful devs and users to help with any problems.
If you are using your system only for MythTV as a frontend then something like Xubuntu or Lubuntu have good lightweight desktop environments that don't contain all the bloat the official Ubuntu GNOME has.
The Ubuntu Server variant makes a good base OS for a backend only machine the only problem is by default it does not have a desktop environment which unfortunately is currently needed to run mythtv-setup to set up the backend. There is work in progress to make it possible to do all the set up using the Services API using either the web app in a web browser or other 3rd party clients but it is currently is not fully implemented.
The only slight niggle with Ubuntu in recent versions is the switch to using snaps for Firefox and Chrome. The snaps get updated very often and if like me you always have Firefox running on the desktop it can't be automatically updated so you get lots and lots of annoying popups telling you to update the snaps
To get rid you have to quit Firefox then run sudo snap refresh
then restart Firefox. It happens just frequently enough to be annoying