Yes, Kublai is a threat--but so is every other civ around. Of the three neighbours, he's the easiest to get along with, in my experience. In the long term, his traits are not that formidable; if he doesn't strike by mid-game, he's not much of a threat. I'm willing to live and let live... unless he backstabs me, which I will prepare for, and if he does...

Consider that if the southern 2/3 of the continent is mine, I can focus my veteran units along the border with Mongolia as a deterrent. I'm already planning on warring more than I originally thought I would be (with these neighbours, can you blame me?). So I'm interested in seeing if I have the diplomatic chops to keep at least one neighbour content with peaceful coexistence.