User Synchronization has nothing to do with username, name (single input), email address, or password as those are entirely core Joomla. User Synchronization just ensures there's an _comprofiler table row for each _users table row so the two tables are in sync. Even if out of sync they should still be able to login (may say they're unconfirmed or unapproved though). I've no idea why your passwords were rejected, but it wouldn't have anything to do with CB as the validation is done by Joomla.