If you look at where Mongols succeeded and failed (notably they didn't control Northern Russia directly, rather keeping them as vassals/raiding occasionally - but they also +- failed in Indochina, India, etc.), it's all about terrain. Logistics of the Mongol armies became much less tenable in non-steppe terrain... I was actually wondering what would have happened had Ogedei not died when he did, as far as Europe is concerned... but then, the Eurasian Steppe mostly ends around modern Ukraine :)