I've a RPi 3B+ here and tried to run Ubuntu MATE 18.04.2 Bionic (ubuntu-mate-18.04.2-beta1-desktop-armhf+raspi-ext4) on it. I flashed the image with BalenaEtcher and inserted it in my Raspberry but instead of a bootscreen I got the error 'Input Signal Out Of Range'. So I changed the hdmi_mode value in the config.txt and tried some values which my monitor should accept but nothing changed. Raspbian is working fine on this Raspberry so I think the fault is with Ubuntu Mate.
Maybe someone has an idea how to fix this issue.