I've been told a few times that Business Units in CRM 2011 are "tricky" and shouldn't be set up lightly since they have irreversible consequences for a CRM 2011 implementation. On the other hand, teams in CRM 2011 seem much more flexible in managing record security. For what reason would I still choose to set up Business Units in CRM 2011? What can I do with Business Units that I can't with Teams (and vice versa)?