I have the same issue running Ubuntu-Mate on Rpi 3B+. It is unable to boot at all.
Regarding performance and CPU architecture mentioned in this thread.
The original RPi was built with an ARMv6 CPU and yes, it is extremly slow. It is a really outdated architecture now.
(It was an ARM11 core which originally used in higher level mobile phones. NOT the smart ones.)
RPi2 is built on a newer SoC with quad core Cortex-A7 (this is ARMv7 architecture, 32 bit).
RPi3, the original one, is built on a SoC with quad core Cortex-A53 (this ARMv8 architecture, 64 bit).
The new RPi3B+ is also using the same CPU cores (Cortex-A53) with a bith higher clock frequency. So I do not see any reason why the Ubuntu-Mate image can not boot on this platform.
Certainly the SoC is different so ther might be other issues, but not the architectecure.
Anyway, I agree that these boards will not replace a desktop machine. But not because CPU performance.
In my opinion 1GB RAM for 4 CPU cores is a joke (a really bad one).
The other problem is the SD card which a real bottleneck. There are other SBCs where eMMC memory is used as the storage for the OS and they show a great performance boost with the same CPU architecture and ammount of memory. I really hope that the next board will have improvements in these two areas. (And also a SATA interface, but not a USB to SATA converter and hopefully USB3.x instead of the 2.0. Even if it not fits in the $30-$40 range.)