At least the RC does not honour the unchecked "Join into the domain". The wizard joins nonetheless.
Yes, checkboxes weren't added to the variables which will be sent to the server. start/join and update/system/after/setup was not in that set. Also values which were False were written as empty string into the profile instead of 'False'.
Seems to work fine. YAML entry OK. → VERIFIED
http://errata.univention.de/ucs/4.0/2.html