Tested Daily (10/02/2020) on old Dell Core2 as Live USB. No issues. So installed over 19.10 root folder (including a format) with home left as-is. Then ran Software Updater.
I had a Brisk has quit unexpectedly message as soon as I opened the menu, which always seems to happen on this device on all recent releases, but I (auto-)reported as Bug 1862623 anyway.
But the upgrade issue I want to report here is probably not a bug per se, but could be an issue for upgraders. I use Compiz (for mouse switching of workspaces and placement of opening windows on specific monitors), and Compiz is no longer installed in Ubuntu MATE.
What happened without Compiz in place was that all windows opened without any windows management decoration. That's hard enough to deal with, but font sizes are affected too (way too large) and lots of minor issues. Solution is to enter MATE Tweak.
In MATE Tweak > Windows > Windows Manager selection was set to [blank] and a message in red displayed that the current window manager was unknown. Selecting either of the Marco entries brings everything back under control.
I would strongly suggest that the current warning in red remains displayed in MATE Tweak until the user's first visit to MATE Tweak, but that - in addition - one of the Marco WM's is temporarily selected for the user. The message could be updated to also indicate this action.
Leaving the WM as 'unknown' doesn't do anything for the user, who is either going to have to select something else or install Compiz without any WM, so it might be better to include a Macro WM in the meantime? It is a trade off decision between making it easier for those not confident in diagnosing and installing Compiz via Terminal by making it straightforward for less experienced users to install via Software Boutique then Synaptic, vs adding an extra step - of changing the MATE Tweak selection - for the Terminal user .