There are some factions that don't have any cities and each army serves a sort of travelling town.

Additionally only available in Attila - horde factions.

That's a pretty cool system, but only available to Rome.

In Rome 2 we got barbarians which are mostly samey between them, hellenic factions which are most common on the map, eastern and desert empires and romans, who by themselves only have thousand iterations of legionnaires, however they also have auxiliary system which gives them access to various units from other groups depending on where you build the auxiliary baracks. That's not so bad when it comes to diversity. Especially the eastern and desert factions have plenty of variety between them. There are barbarians with germanic, nordic, celtic and slavs, there are romans with western and eastern variants, eastern factions with various persian, desert, eastern elements, and nomads with 2 hun variants and alans. While there are a lot of germanic factions there are 4 major culture groups with enough diversity between them to be interesting. I've seen some people complain that there is not enough diversity in Attila and 'too many germanic factions', which is just not true. if you play as romans and convert to paganism your soldiers will still praise the lord anyway. They also don't account for changes in your faction, eg. As for the other 'aristic' side, music is pretty good in both games (I personally prefer the Attila one) while voices show the cheapness of CA with very limited lines and plenty of copypasta between various factions. However Rome 2 family and character ui is really atrocious and unreadable, navigating it can be a pain, meanwhile in Attila it's pretty clear and informative. Personally I prefer Attila grim to Rome 2 yellow, but to each his own. Visual side is the part both games are the most similar to each other. Plus it says something when CA introduces gameplay elements from Attila to Rome 2. It's better than Rome 2 in pretty much every aspect of the game, even after the latest update.

Attila total war soundtrack