Emrakul, the Promised End doesn’t let you control other players’ priority
Normally, the game lets you put stops were you normally wouldn’t need priority to facilitate strategies that require it. when I use Emrakul to control of an opponent, I can still place and use those stops for myself, but I can’t make my opponent hold priority this way. I can’t do things like forcing them to activate an ability during their untap step or cast one of their spells then use their own counterspell on it, because priority automatically passes from my opponent to me after casting the first spell. This hinders Emrakul’s ability to strategically generate an advantage. I don’t know what it would take to fix this, but it seems worth looking into, so Emrakul can hopefully work the way it’s supposed to.