I've been playing 5e for 5 years and have not found stunning strike to be a problem at all.My personal position, previously stated in this thread, is that Monks aren't particularly underpowered, but that Stunning Strike is overpowered, and is particularly a problem that the other uses for Ki don't compare well.
Also, this logic again can be used for anything - Sneak attack is so overpowered other uses of actions (shove, grapple, search and at low levels hide and disengage) don't compare well