Maybe if they had some Elephants or Chariots or something to fight the Parthian Catraphacts or the Egyptian Chariots
This makes it impossible for the AI to survive as Seleucid Empire, it is not impossible for a human player to win, but still, it is not balanced and I think it should be in Rome Total War II. This is not due to its units, but to their initial position, which is surrounded by several deadly enemies such as Parthia or Egypt, moreover Armenia with their horsemen and Pontos. Well, as you know, the Seleucid Empire is one of the weakest factions in RTW 1, if not the most one.