I agree. That mines do help, however they cost a fortune! It was not a problem with Rome where economy can surely substantiate it, but it is a problem here. My only real source of income comes from exterminationg populance when I conquer a city... all teh money from cities go towards paying for army, most of which are regulr cheap archers and spearmen that I need to maintain order. I think it is rather imbalanced.
May be the intention was to never allow you to become an empire if you play as Parthia. Then it malkes sense if you never have more than 10-15 cities...
As for horse archers....I do not have that many of them, they are not as expensive, but they are my primary unit. I do not have armoured hourse archers.
As for bonuses, culture, and distance I think they should be offset slightly.
In fact... I often have this situation: a settlement has only 500-600 inhabitants, I have 10 units in it, and still it revolkts... nonsense in my opinion.