Figure out this one: When conquering Denmark (or sweden, norway) in Early, you cannot build Vikings (since they are faction specific) as a foreign power, but you cán build Viking Huscarles, Viking landsmenn, Viking thralls (? ?)! (since they are region specific)

The funniest example: you cannot build Horsemen, since they are again, faction specific! But you can build VIKING RAIDER CAVALRY, since they are region specific. RRRight..

Worst you can have is like Swabian swordsmen: faction AND region specific.. for HRE and Swabia respectively