The only thing Mongolia has going for it is the Keshik, and after they've outlived their usefulness, you are stuck with mounted/tank units. Lots of promo's, sure, but you cannot finish a game with cavalry/tanks alone, where as with highly promoted Gatling/machine, you can.
There is a reason why China is often cited as one of the most powerful Civs since vanilla and Mongolia rarely gets mentioned. Keshiks are a fun medieval rush and very powerful on cooked maps, but the archer line is simply more useful in the big picture.