Drifter's ultimate is a devastating crowd-control effect that prevents you from engaging enemies outside of your line of sight. If you're an M1 carry with plenty of souls but can't apply them in teamfights because of a CC effect, you'd naturally pick out unstoppable and activate it in response. However, even though Drifter's ultimate is a highly effective CC effect, Unstoppable does not cleanse the effect and you are still unable to shoot at enemies outside of 15 meters for the next 10 seconds or so.