Might add this to the list as I believe this is rarely seen.

You can only support a friend's raid requests up to 3 times daily regardless of difficulty (eg. 3 Rag -OR- 1 Expert and 2 Rag -OR- 1 Rag, 1 Ult and 1 Expert). Limit is usually bypass if they are same Union member (count as union help request) and you answer the request of another friend involve in the same raid a.k.a Friend of the Friend who requested for help. It happened so rarely I forgot it even existed till I bump into it today.