Anecdotally, people who routinely use Revo Uninstaller seem to have run into a lot of weird problems with their PCs over the years. Could be coincidence, though.
Installing 10 over the top of 11 should work, but you may wish to manually delete your configuration folders as well as the main Directory Opus folder to ensure things aren't left over.
To find the config folders: How to backup or locate your Opus configuration (Simple)