The nomadic peoples are interesting to play, though most barbarians have very little unit variety.
I wouldn't say that and everyone of them have special units like Chosen Archers, Chosen Axemen, Lombard Archers, Night Raiders, Lombard Berserkers, Levy Spearmen (not the same as the Franks), Burgundian Lancers, Barbarian Noble Cavalry (Alemanni, Burgundii, Lombardii) and the Franks has these http://www.twcenter.net/wiki/Franks_(BI_Faction)#Unique_Units plus the Saxons has these http://www.twcenter.net/wiki/Saxons_(BI_Faction)#Units. http://www.twcenter.net/wiki/Celts_(BI_Faction) will fight with old fashioned units, but they're effective since they have chariots and berserkers in their arsenal they can recruit.
Both Celts and Alemmani can't from a horde and are the only barbarians who must defend their last settlement with all they got, because if it's lost the game is over.