KH has it tougher than Macedon or Epirus, because your starting army is both smaller and has less advanced units. Also the Macedonians have pike phalanxes and superb heavy cavalry that can smash though almost anything, even your elite Spartan bodyguards have trouble with them, while KH has no cavalry at all. Finally, AI Macedon can replace its losses in dead soldiers, whereas you, as KH in the early stages can't recruit anything because you're in debt, so only your FM bodyguards are replacing their losses.

Casse is indeed very easy, as long as you get their economy running properly. Casse don't even need to blitz any cities to get their economy making a small profit, so you can just sit there and spend your money on new buildings for your capital. Also Casse get lots of new family members quite quickly, and six FMs make a formidable chariot-based army on their own, even without any infantry - enough to raid enemy towns and kill off more of their troops with each raid.

I tend to regard Casse's victory condition as just to conquer all of Britain - once I've done that, Casse are so powerful that's it's really not much of a challenge to invade Gaul, because they are so rich by then that they can afford unlimited mercenaries. Casse have everything they could possibly want right there in the home islands.