I think that’d depend on mode.
I find that Ana in QP wins when you overwhelm and looses when you attrition.
For example, vs Mercy
Ana massively out burst heals Mercy and can disrupt Mercy’s heals with Biotic Grenade and can possibly take a player out of the fight via sleep.
The thing is though many of your abilities have cooldowns and you have to reload. Mercy doesn’t have to aim and Mercy regens health both of which are luxury’s you don’t have. Mercy’s res also depends on the fight breaking enough to give her space/a window.