I do not use Bzr but Mercurial and Git.
And as far as i know using both a sync service and a revision tool on the same folder may create problems especially if that folder is updated by on several computers running Ubuntu One (or any other file syncing service afaik).
I've been having a few issues with ubuntu one and mercurial and i think it has to do with the fact ubuntuone and the mercurial repository might not be in sync depending on the sequences of changes and commits. For instance I might update files on one computer and not commit them or get updated file from ubuntuone before or after i pull changes.
I think a second reference point (the bzr/mercurial/git) repository is enough to confuse ubuntuone.