Ana is NOT! a sniper.
She can perform the healing role just as well as Lucio/Mercy or even better situationally since her single target healing output is by far the highest in the game.
But she is also highly dependent on team composition, if your team got plenty o tiny hit box heroes that spam spacebar all day long even when their not even being attacked your better off swapping to Lucio or Zenyatta.
Notice how i don't say Mercy?
That's because i think Mercy is overnerfed now, i perform better with every healer then Mercy now and Mercy was my main for a long time.
With ana i can keep up a Reinhardt against a whole enemy team blasting away at him since she has like 160hp/s on a biotic grenade target that is further boosted by Zenyatta/Lucio's HoT.
There's no way Mercy's mediocre healing can do that.
The 30% ultimate nerf just killed Mercy's usefulness. Her healing output is low compared to Ana/Lucio and in a pickup both Ana/Lucio has more tools to protect themselves with Sleep Dart/heal grenade and knockback/boost.
Mercy need a huge boost to her damage capabilities for her to be on par with the other healers, as it is now shes just inferior to them all.