its not their numbers thats so much of a problem(heck, 2 20k stacks with godly bonusses would even be ok), however the fact thats it about IMPOSSIBLE to get a counterarmy in a single rovince makes fighting them near impossible.
The fact that the current solution is "get 120k troops, then find a way to converge at once!" is a problem itself. I don't like that the only way to play the game in Persia or Russia is to blob to the most ridiculous size possible just so you can fight off the Mongols. I'd much rather see a system where a player doesn't feel like his game is lost just because they can't beat the Mongols one on one. For starters, the AI needs to be better than it is now in regards to vassalized conquered holdings against the Mongols. They've improved, but while the Mongols may not be quite as apt to start revoking all the Russians without cause as they once were, there's nothing stopping the Russians from throwing their lives and lands away in silly, uncoordinated, hopeless rebellions (or from just getting squished by their Mongol neighbors inside the Horde).
1. Getting conquered by Mongols shouldn't be a death sentence. Rebuilding your strength from within and breaking free should be a realistic scenario.
2. Like EU, wealthy realms should be able to keep the Mongols at bay with tribute. Not just gifts, but an actual mechanic where Mongols will be very likely to leave you alone so long as that steady flow of gold is trickling into their hands.
3. Additionally, willingly handing them land should buy you a few years, maybe a decade, where they'll leave you alone (assuming they won't accept your tribute).
And while I'm happy with the terrifying numbers of Mongols, and their equally terrifying horse archer swarms, I still absolutely loathe the 120k+ doomstack. Your only hope is for them to split it up (which they'll sometimes break 20k-30kish troops off of it)... otherwise, its a silly "I win" button, resulting in the feeling of necessity blobbing, which the AI is mostly incompetent at anyways resulting in an all too frequent "Russians as an endangered species" scenario. In my current game (as Bulgaria, so I've been helping my Russian brothers out), the only reason the Golden Horde lost their doomstack? Because the Ilkhanate converted to Sunni, invaded, and their doomstack won the battle of doomstacks.
That's all I want: Mongols that don't drop the 120k death bomb on everything, and players in the east not feeling they have to blob by any means necessary as the only approach to the game. Yeah, okay, not that simple... but I have faith in PI to eventually work this out. As it is, the Mongols have come a long, long ways from how they were at release (lol at France having inherited a single Russian county, and that resulting in the Mongols invading France).