Ana's overall kit is simple enough to learn, but she is one of the most difficult supports to fully master. All of her abilities require some sense of precision, with her Biotic Grenade being more forgiving. Even if the ability is AOE, players still need to know the flight speed and drop-off of the grenade to not miss. Ana is a risky pick, but if played to her maximum potentially, she can essentially be one of the best supports.
One key reason Ana is not at the top of the list of support heroes is the difficulty gap between her and other supports such as Mercy or Lúcio. Since all of her abilities, even her primary fire require pinpoint accuracy, missing a couple will be detrimental. In a perfect world, Ana can hit all her shots, allowing her to have the most heals in the game. Since this is impossible, Ana is overshadowed by more consistent healers.
Mercy has two aspects over Ana, a more consistent stream of healing, and much better mobility. These two allow mercy to pump out more healing over the course of the game, as well as being safer than Ana. That being said, the Nano Boost buff is a great starting point for Ana, even though she does require some more. The Mercy nerfs on the PTR will bring her down a notch as well.
The extra 300 health will allow allies to jump into the enemy team with much less fear. Ana does not have to worry as much about healing the Nano Boosted friendly and focus on other teammates. This gives Ana more breathing room to utilize her entire kit. Not having to worry about the Nano Boost target will allow her to get a couple of more shots in, whether it be at healing her teammate, or taking down an enemy.
This buff should put her on par with the more consistent healers, allowing players to change up their play style a bit and not tunnel vision on the Nano Boosted target the whole time. Top Ana players will see this buff benefiting them greatly since their aim is already fantastic. Building up the ultimate charge will be much fast, which they can use more generously since it's essentially another quick burst of healing.