Custom entity limitations

This is a summary from the book Working with Microsoft Dynamics CRM 3.0.

A list what you can't do with custom entities:

• You cannot create many-to-many relationships between entities. Well... you cannot create many-to-many relationships at all
• You can not merge two custom entities together
• Export and import is possible only by using some third party tool. Microsoft CRM Data Migration Framework does not support custom entities
• The Microsoft CRM system entities include a relationship to Customer in which users can select an Account or a Contact. For custom entities, you can specify a relationship with the Account entity and the Contact entity, but you cannot create a relationship to the composite Customer entity (in which users can select an Account or a Contact on a single lookup).
• Only one parent relationship per custom entity is allowed
• Custom entities don't appear in an entity rollup
• Organization-owned custom entities can't participate in Microsoft CRM workflow
• You cannot create a custom relationship between an entity and itself
• You could not create relationships in such a way that a form has two or more lookups that connect to the same custom entity.
• Custom entities cannot have a parental relationship behavior to system entities