Her healing output it's not quite as high as other healers.
Hers is a burst like thing that cannot be substained over long periods of time.
The energy bar limits you there.
Mercy heal is fast and limitless. She is very well suited to defense scenarios when your team is packed around choke points.
I can't keep a Reinhardt going melee alive just by spraying on him in the way Mercy can.
It's true that I get gold heals many times but I always have to make the choice between finishing off the enemies quickly or healing my team mid-battle.
I feel like many fights I just can't heal fast enough. So I focus on the enemies healers or annoying enemies and THEN heal my teammates.
Heal and DMG orb is always a hard choice. Do I add more DMG to finish the encounter faster and prevent DMG by killing enemies ? Or I throw heal orbs so my teammates get an instant burst healing even if I'm unable to keep them full health ?
Then again, her blink and ult combo are amazing. With her beam you don't need to make the choice, you can both finish enemies quickly WHILE healing your teammates.
It fills up really quickly too, so I can support my team more effectively during teamfights in the payload.
It's not Lucio levels of substain, but they come quite close.