Does Ubuntu mate work well with the pi 4 64 bit OS, I’m wondering since it’s not raspian does the GPIO portion of the board work well as well as the graphics card drivers etc, is there any downside of using Ubuntu mate over raspian,
If you mean the Raspberry Pi 4 hardware specifically, then it's not supported at the moment. Watch this topic for updates:
I've heard that current images designed for the Pi 2 & 3 models are unlikely to boot on the Pi 4, however others may have experimented in the meantime.
I believe the Pi images use hardware acceleration where possible. Others who use Ubuntu Mate on the Pi frequently will be able to comment further.