Again, not borne out in play. Big secret that most wizards know is that monsters have doo doo saves. Sure, dragons or giants or other meaty critters might save, but you've got enough ki to take it to them round after round - everyone rolls a 3 eventually (typical D&D fight is about 3-5 rounds, so it's not like you need to conserve your resources much there). I've seen monks lock down enemies often enough that house ruling Stunning Strike is one of the few power-based house rules I've considered for 5e, due to it turning so many 4v1 fights into cakewalks for the players (didn't end up doing it, because I prefer to allow power to breathe a bit, but it wrecked some fights like arcane eye wrecks some dungeons!)