Seriously. I played a monk, and while Stunning Strike came in clutch sometimes, it was balanced by (a) depending on the opponent failing a save, and (b) using up ki, which will run out over a longer encounter.

Plus, as a DM, I never really had a problem with the players pulling off a huge upset like that. 5e is heroic fantasy, so let them be big damn heroes sometimes. They won't always be! (Though I do play up as though I'm shocked and frustrated, but that's just to play the heel and let them feel extra victorious. :P)