Strange things can happen. I've had the Koinon Hellenon succeed more often than the Makedonians. Similarly I've seen the Saka Rauka become modestly successful, and the Arverni overcome the Aedui (rarely). But I'm yet to see Arche Selukeia beat the Ptolemaioi, I think most people agree that's an issue. My Ptolemaioi campaign is more "moderate" in difficulty, after the first 2 big battles it's a piece of cake.