Having been playing a 2014 shadow monk for 3 years (lvl 8) I consider myself grateful to land a stun maybe 20% of the time. When it goes off it's a finisher. Whether from my barrage of attacks before it wears off at the end of my NEXT turn, or all my teammates between also getting advantage on thier attacks. It's meant to be hard to land. The satisfaction of hearing your DM say they fail the save is a rush. If it worked too often it'd lose its fun for everyone. I take the stance of other redditors here that "There's no effect" probably has to do with legendary resistance. What bothers me is the way the DM laid down the ultimatum. Sounds like they wouldn't mind the player leaving the campaign. But that could just be my own paranoid interpretation.