In LotR, Mordor is presented as the very powerful war machine with seemingly unlimited troops, whereas Gondor is becoming weaker and weaker. Now, how are you going to balance that?
Of course, game balance is important in any mod/game. However, here it seems a lot harder simply because Mordor and Gondor are so close to each other. Same goes for Isengard. How are you going to make it so the little Isengard doesn't simply get crushed by Rohan when the game starts? Or the other way around, how are you going to prevent Isengard from steamrolling Rohan from minute one onwards?
I don't think anyone wants to see Gondor conquering the Gorgoroth 10 turns in.
The problem is, if you make Mordor strong enough to defend against Gondor (read: giving it enough troops to be a challange for the human player as an adversary), a human Mordor player might use those superior starting troops to conquer Minas Tirith within 4 turns.
Maybe garrison scripts could be used to give e.g. Gondor the chance to defend against an invasion while not being able to steamroll everything? Broken Crescent has a lot of success with this feature, maybe it could work here, too. It would also fit with what happens in LotR, Minas Tirith is sieged and lots of people come to defend the city against the invaders.