I find Mercy can be AMAZING at certain choke points. You can safely keep your whole team topped off on defence as long as someone deals with flankers. Had a few games where I switched to her at a choke point and kept people alive for a long time. Once they're past that point I go back to Lucio who's a bit more dynamic.
On attack, I find it much more difficult. She's too easily picked off if people aren't looking out for flankers.
If there's someone like a Hanzo or Pharah, along with someone like D.Va or 76 it's nice and easy to fly between them all as they are all near the main group, but generally in completely different positions.
Mercy also shines when your DPS is full of flakers. Lucio can't really keep them healed, so I find it's better to be Mercy and keep up the tanks and when the flankers come running back with their tail between their legs, top them up.
On attack, I find it much more difficult. She's too easily picked off if people aren't looking out for flankers.
If there's someone like a Hanzo or Pharah, along with someone like D.Va or 76 it's nice and easy to fly between them all as they are all near the main group, but generally in completely different positions.
Mercy also shines when your DPS is full of flakers. Lucio can't really keep them healed, so I find it's better to be Mercy and keep up the tanks and when the flankers come running back with their tail between their legs, top them up.