Hi Kye!
Indeed you are right, not easy to get an overview. Actually you summarized it pretty well already and due to the AOR system the differences may get blurred even more because a faction with weaknesses in one category can compensate with AOR units. But thats a good and very intersting thing I think.
Rome e.g. has also the advantage of their auxilliaries which gives them good versatality.
Pontos I would say has its strength in the defensive like most greek factions. But I didn't play them myself and neither fought against them too often so I might be wrong. (they also have chariots don't they?)
Getae have some decent units with good attack values.
Arverni like most Celts do have some good cav but also decent sword inf. Compared to Suebi I find it easier to fight against Celts than Germans. Suebi have really good archers and cav they are hard to fight against.
Like the British tribes, my currently most feared opponent

British units have a tremendous charge and all their units seem to have very high attack values. Not to mention their "uber"-slingers with a defense value of about 80.
Those are just some observations of mine so I might be wrong. I never played any barbarians myself so far. But I would be very interested in someone telling me how to fight British armies in the most efficient way because they are really giving me some trouble esspecially when they come in numbers (and they always do

)