Ubuntu MATE 18.04 still hasn't been released yet for the Raspberry Pi. Even worse, the current RPi image of Ubuntu MATE 16.04 is out of date so you'll have to upgrade all the packages on your system after installation. If that doesn't sound bad enough, the current image doesn't work on the Raspberry Pi 3B+, released in March 2018, and the Raspberry Pi 3A+, released in November 2018 because the bootloader firmware (bootcode.bin, *.elf, *.dat) is out of date. A number of steps must be taken to be able to boot the current Ubuntu MATE image on these models.
Are there any updates on the development of an Ubuntu MATE 18.04 image for the Pi that boots on the Pi 3B+ and 3A+?
I think 18.04 should use the Ubuntu-provided linux-raspi2 kernel which actively receives updates unlike the Raspbian kernel which you must update manually. Raspberry Pi PPAs should be ditched, they make the Ubuntu MATE image look more like a 'Raspbian based Ubuntu' instead of a pure Ubuntu installation that functions exactly like the desktop versions.
The Ubuntu MATE, Xubuntu, and Lubuntu images for the Pi were made using a tool called the Ubuntu Pi Flavour Maker. This tool uses debootstrap to install a minimal Ubuntu system which is then chrooted into and the Raspberry Pi PPAs are added and the necessary packages are installed, and then oem-config is setup so that on first boot, a setup wizard will load where you can configure your user account, etc.
I created my Debian and Ubuntu images for the Pi 3 in a similar way. I used debootstrap, but decided to skip oem-config and setup a default user account instead. I think the biggest mistake I made in both my Debian and Ubuntu images is failing to include support for hardware accelerated graphics. A big mistake I made in my Ubuntu images was using raspi3-firmware which includes old bootloader firmware which didn't work on the Pi 3B+ and 3A+. flash-kernel detects your platform when updating the kernel and copies the vmlinuz and initrd to the locations specified in the database. raspi3-firmware also does this and overwrites config.txt as well, causing you to lose all modifications you made. flash-kernel and raspi3-firmware are incompatible with each other.
If anyone wants to help create Ubuntu 18.04 images for the Raspberry Pi, then good luck!