I think the HRE in Medmod 3.14 is the only faction I have found that is hard even when you get good. All other factions are easy enough if you are aggressive early on. Only with the HRE does that strategy not work. With them you need to protect yourself from about 5 simultaneous attackers while you slowly take them all over. It stays a challenge for quite a few years. I think in th XL mod, the Armenians are a little tough but probably not any more so that the Aragonese or Serbs.