sure. it is possible to win. but still expect to lose some territry. they arent some kind of lstorng enemy like HRE that should be properly balanced. they are the historic menace that obliterated the eastern regions. while its perfectly possible to beat them the first time, theyre ridiculously strong and SHOUDL me near impossible to defeat,
Exactly. The only reason you shouldn't is if you're being a super gamey type who refuses to actually deal with any kind of adversity in CKII and you've planted the entirety of de jure Russia's army on the map's Eastern edge long before the Mongols actually arrive. If you can man up and avoid that, then from personal experience I'd say there's a good chance you lose Perm before you can even organize your armies fully. If you haven't been playing a super-aggressive game, then they'll be the challenge they were meant to be.
With that said, I did beat them with Nubia facing their first invasion (Ilkhanate). They had nearly three times the army, so I just loaded everyone onto boats and moved along the coast picking off the smaller Mongol armies while avoiding the stack. Of course defending the coast of the Levant is a lot easier thanks to this than defending inland Russia.