It is not unreasonable to ban a specific strategy from all formats due to being uncompetitive, including a banlist. An example is funbro.
Yeah, is Mega-Gar Uncompetetive simply cause of his ability? there is Gothitelle and Wobbufet, and they are not only not being banned from Ubers, they kinda suck too.
No. He's uncompetitive because not only can you not switch on him, but he has several things he can do to take advantage of that.
1. If against a sweeper, he has base 130 Speed (faster than all remotely viable Ubers except Deoxys-S, Deoxys-A, Deoxys-N (assuming they run equal Speed; they might not always want to; they're also frailer than he is) and Mega Mewtwo Y, who's the one real counter to him) and base 170 Special Attack, so he most likely outspeeds and OHKOs with Sludge Wave or Focus Blast.
2. If he's up against a wall, he Taunts and then Perish Songs, waiting out whatever their attacking move is until the last turn, then switch out and get the free kill. --- Generation (pi)(i). When you see this, raise e to the generation and add 1. Social experiment.