Epeiros sucks in the empire phase, because its government-types are not the best. They are too barbarian. They don't have the perfect mix of Hellen culture, ferocious strength and royal appeal like Makedonia.
Actually, Epeirote government is quite good. Better than the Satrapeia-Hemi-Autonomos and its whooping public order penalties at level three, while at level II the bonuses fairly outweigh the public order malus. If anything all Epeirote governments provide better benefits than their Successor counterparts, regardless of the first time appearances.

"Royal appeal", I guess no one beats Pyrrhus here ;~p. Otherwise, the Epeirotes are a fairly "plain" Hellenic faction, which makes your remark about "barbarism" a bit far off, unless it's just plain old Athenian bias .