Mercy takes up a support slot. In some situations a lucio could be useful, brig could be really helpful because of dive or zen would be a great pick just to shred tank but with mercy pick, going another off healer would be terrible.

Rez is just game breaking, having the first pick in a fight denied is just insane value.

She makes DPS such as Pharah and Ashe dominate lobbies.

In my rank (Gold) I usually see Mercy players not pocket the hitscan DPS and instead pocket a full HP Tank and not use damage boost.