The reason why Pharmercy is so dominant in low ranks is less about mechanical skill and more about a lack of gamesense. The Mercy is stuck with the Pharah, usually flying beyond or at the frontline. Their other support is extremely vulnerable running a Pharmercy. Playing dive comp against Pharmercy just means focusing their insanely weak backline and then forcing the Mercy to disconnect and heal the rest of her team up, which usually ends with an easy clean-up.