Please see the answer from /u/Tiako on a similar question an year ago:
It isn't crackpot on the order of Ancient Aliens or Gavin Menzies, it just doesn't have much to support it. Pretty much the entire argument rests on a phonetic link of their names, which are exogenous. Against that is a very large gap in time and space, and rather distinct cultures once you get past the "scary horseman" bit (for example, the Huns scarred their chins to stay clean shaven, the Xiongnu wore beards).
It is not impossible, and the archaeology on both ends is too underdeveloped to make definitive statements, but for now the theory must be confined to speculation.