Stash only uses its ability when you reshuffle. Unless you have a Mystic in hand or in play when you trigger the reshuffle, how will you make your Stashes line up properly?
Sure, you can flood your deck with Mystics and Stashes, but if that's your goal, isn't a Silver flood almost as good and significantly easier to achieve?
If 'X' is a Chancellor variant, then fine--but that's Chancellor/Stash, not Mystic/Stash.
But it's possible that I'm missing something.