Quite a lot of people have done this. My vague recollections are: no avgas in Moscow, and you need the interpreter if flying VFR in Russian airspace.
Once going east of what we might call civilised Europe, published airport data gets less reliable so one needs to sort out the PPR / avgas situation carefully. I planned a trip to the Ukraine (Odessa) in 2007 and found it OK with a long range plane but extracting info from the airports was like pulling teeth. But completely doable.
I think all these trips are easy flying-wise. It's the planning / logistics that take the time.