Priority confusion is not a problem. Hitting yourself because you are confused is not a problem. Prankster + [INSERT STATUS MOVE] is not a problem. But you are purposely stacking the odds against your opponent AND you have a way to abuse those odds. Foul Play is a legitimate move on Klefki. If this ban were to take effect, who in the right mind would use Swagger over Foul Play? I asked a few pages ago if this ban was too complex to be enforced. I still would like an answer to that question.
Klefki isn't the only pokemon, you know. Foul Play is a legitimate move to get advantage of the non-haxy part of Swagger. So Foul Play is not the problem. If you allow Swagger+Prankster, however, there will still be a big enough group of people that will complain of losing control of the battle. And no, that ban is by no means too complex (just see the history of the bans of Endless Battle Clause), but it's not the right thing to do.