In a long campaing, Egypt must be fairly hard too. They have lots and lots of crummy units, their best infantry are 2H axemen who will not try and hit horses (OK, that's a bug, but still ), and you'll have to fend with both lots of crusaders AND the Mongols/Timurid by the time you're done. And, you know, the über-Turks too. They are also completely inadequate in the gunpowder era, be it arty-wise or musket-wise...

Sure, they have on of the best starting positions and roll in the dough pretty early, but that only goes so far.

Poland is actually not that hard. If you manage to get a wall of castles to the West early on (Szekelzln (can't remember the name, that 0% christian castle next to Hamburg),Magdeburg, Vienna) and keep them properly garrisoned you'll be able to fend off anything thrown at you from your Christian side, meaning you only have to worry about your Russian front, and seaborne invasions into your city-heavy heartlands.
Plus of course, Poland has awesome units (mounted Polish nobles are just sick, dismounted ones are very solid spearmen very early, Lithuanian archers have stakes, good horse archers to give you the range the Lithuanians footmen don't have... all in all, a solid roster)