I vote ban because Gengar-M have an ability unhelthy for the meta that give he the possibility of remove counters of the principal sweeper of your team such as Arceus or Xerneas and thanks to it you can sweep after a SD or Geomancy.
Gengar-M have a great movepool that with his ability help a lot for example he can use Taunt+Destiny Bond for force the rivals pokemon (Arceus,Yveltal Def,Scizor...) to attack you and die in the procces, this movepool also give he the oportunity of trap walls such as Chansey and kill it thanks to the acess of Perish Trap.