Most factions have a difficult start (of them Hay is probably the most difficult) but become much easyer when the initial fighting is done, or the crisis is over. There are some that remain challanging.

With the Arverni, for example, I had severe problems to get a profitable economy even after the conquest of Gaul. That meant smaller armies with units that are in no way better than the units of your greedy neighbours (Rome and Sweboz). The same would be with the Aedui, even though I had never tried them.

I would consider Saba as very hard too, because once you have conquered the four towns around, your only way to expand is attacking the Diadochs.