Last I knew the balancer for a large two-team game broadly works in the following way:
First, it attempts to find two balanced teams, putting all clans and all parties together.
If it fails to find a balanced set of teams subject to that restriction, then it balances the teams ignoring ALL clan and party considerations.
I do not know what the threshold for a "balanced enough" game is. It is probably pretty restrictive. Paraphrasing the Law of Large Numbers, for a large teams game there are likely to be a lot of team arrangements that are very close to even in terms of rating.
I do not remember exactly what influence rating variance within teams has on the balancer.