Personally, whenever A suspect test comes up like this, I always find myself seeing how good it does on my own team. So, I built a standard NU team that carries Liepard. I built two different liepard sets, and tested them. One which was SwagPlay, and one where it ran assist. The SwagPlay set worked decent, it was a 50/50 result, as the set was based on luck. Did they hurt themselves in confusion when I wanted to attack? Thats basically what the set rode on. My assist set worked differently. With some of my pokemon carrying moves to setup, and others carrying a variety of moves, assist worked out well. It seemed to choose super effective moves most of the time that I used it, and it allowed me to sweep some teams. It did have some downsides though, sometimes not choosing good moves and screwing me over, and most of the time, thats when I lost.
Running assist + prankstar gives priority. That is why it is used, so if you can get some attack or sp.atk stat boosts on liepard through baton pass (I used ninjask), then have it spam assist, it will always outspeed you, and smash you with a massive hit. I found it to be broken.
Personally, I think that both sets are based on luck, however the assist set seems to have a higher chance of sucseeding. Prankstar is a gimmick that was uneeded, however adds something to the game. All in all, I believe that liepard itself should not be banned, however banning assist + prankstar is an ok idea.