After living a crusade to liberate Poland from the Golden Horde, I agree with you. I though "people" were bound to general rules in this game, but when I saw the doomstack, I felt like we were all doomed, not because the AI had a good strategy, but because it didn't follow the rules. They should have suffered from attrition like I was, it was just not fair.
I used as the Byzantine Empire the trick of fleet-carrying mega stacks to lure an AI Il-Khanate and making it lose half its troops (with 1.07 and CK2+). I thought of it myself, but this is extremely gamey and it feels unreal and coward. Where are the land combats in all of this? It kind of break the immersion and I would like the Hordes to be able to suffer attrition.
Someone suggested the death of the mega-stack at the end of the first Khan. It could work, but it would still be easily beatable. I very much prefer the possibility for the stack to be invincible only when he appears (to simulate the fact that there are land behind it), then once they took control of enough provinces to be able to sustain it they would lose their invulnerability to attrition. They would be considered as a retinue popping of nowhere (of Central Asia) and beating them would be far easier since at least attrition would work on them.