I just trashed a site with UGM going from 2.8.3 to 3.0.0-RC2. Still don’t know why, but UGM hung near the end of the process. The site is in a subdirectory and UGM failed to find one of the processors. I had to move the core back under the web root and modify the config files, so I may have made a mistake in doing that.
I’ve successfully used UGM to upgrade a couple of other sites, but be sure to back up any site completely (files and DB) before trying to use UGM to upgrade.