Hmm... too tough? They're annoying I'll give you that, my problem when I encountered them (playing English) was that they didn't attack my cities so I couldn't fend them off at a siege they just kept going west, so I had to chase them down in the open planes south of Ryazan. Fighting massive amounts of horsearchers in the open field...very annoying. And it doesn't help that they huddle together so that you can rarely attack less than three stacks at a time.

I assume that they don't attack the eastern cities because they are supposed to invade Europe and with the stacks the computer seem to like to assemble in the higher difficulties the massive stacks of the Mongol's wouldn't get far if they meddled with the computer first. I captured both Sarkel and Bulgar before they could get to them, but didn't have the forces to defend all river crossings :/ the ones I held along with the settlements were ignored, probably a smart move with English longbowmen on the defending side. ;)

It's the only time I've encounterd the Mongol's though, I played the Danes after that, but I had tired of it by the time they emerged in Bagdadh, which was the Turk's last settlement and the Egyptians had loas all coast settlements and then some, so I took Jerusalem and was done with it.

Trying Milan now, fairly sure their insane pavise crossbow militia will make shischkebab of any horsearcher army though. The danes would probably have had a much harder time, especially in the desert, the War clerics heavy armour may make for good protection from arrows, but not from heat...