I posted earlier about some issues with UpgradeMODX when upgrading to MODX 3.
I cloned Bob’s Guides and ran UpgradeMODX to upgrade from 2.8.3 to 3.0.0. I had to move the core back under the web root, and adjust things for having the site in a subdomain. All of the issues I ran into turned out to be caused by configuration errors I made, and plugin extras that didn’t work in MODX 3 (some of which were left over from incomplete uninstalls).
As far as I can tell, none of the issues had anything to do with UpgradeMODX and the site is working fine except for the incompatible extras like Articles.