I have no problem with tech tiers and with unit advantages for the West, but it may be that the advantages they get are just too large right now.
I think the advantage of Western over Eastern and Muslim troops in the late game should be about 3:2. The Ottomans fielding 30,000 troops should be about the equal of 20,000 Western troops, 150,000 Russians vs 100,000 Westerners should be an even fight all other things considered, especially since Russia gets such monstrous manpower advantages.
And as far as African troops go, they should need no more than a 5:1 numerical advantage to fight a Western European arm on even terms. The Battle of Adowa should be the prime example, the Italians sent about 15,000 troops if I remember correctly and got smoked by 100,000 Ethiopians. If in-game those 15,000 Western troops could blow away 100,000 Africans, then there's a problem.