Did you try trolls to fight upgraded armies? With the Nazgul Debuff, the Gothmog leadership and some orc overseers your armies also shouldnt fall behind completly. Of course, orcs arent as strong as upgraded Uruks, but the combination of your heroes and some special units like trolls or Mumakil make Mordor also really strong later in the game.
Your own heroes... well, I guess you can make them weak or overpowered, depending on your skill choices. It is not really a balanced feature at the moment
I was wondering about mentionning that : I did find trolls (with armour and sword upgrades) to be very effective against enemy troops, but it kind of feels like you just have to field as many of them as you can, with a swarm of random orcs to have a fighting chance : all in all, I felt somewhat limited in my choice of useful troops. It's especially true if you don't manage to capture/keep an outpost, because then your only strong trainable unit is the troll : as such, in both power and diversity Mordor feels much more dependant on his outpost buildings than other factions.
I feel that maybe giving more importance to Southron/Haradrim could be a way of filling the "gap" : if orcs are mindless bow-fodder, it would somewhat make sens to have black-men be the better troops, maybe even with upgrades.
Another thing : I agree that in the movies orcs are the worst possible soldiers, only useful because of their vast numbers, but I don't remember it being that much the case in the books : they're not very disciplined for sure, and weaker than elves/dwarves/humans, but still...As such, I don't see why threre couldn't be some decently strong orc units (veterans maybe, or guards of the black gate or someting ^^).
Lastly : I remember some Mordor troops that gave the rohirim a tough time on the fields of Pelenor were half-trolls with great axes (I think that was it ^^) : not to mention there aren't many units with axes and it could be very cool (barring the very ugly half-trolls BfME II gave us for Misty-Mountains) couldn't they fill to some extent the somewhat vaccant spot of strong non-pike melee units for Mordor ? : P.