Ana does 80 dmg per shot, being able to shoot 1.1 shots a second. Ana has a potential of dealing 88 damage per second (assuming with infinite amount of shots). Lucio without "Amp it Up" does 12.5 healing per second. Amp it up healing does 40 per second. 

Lets just use Lucio as an example. He has 200 health.

Killing Lucio would take 3 shots without amp it up. With Amp it Up, it would take about 4-5 shots since Amp it up would lasts 3 seconds long. 

However your Biotic grenades allow you deny any heal your enemy can get from Lucio. So I'd say no, it's not useless. 

Edit: There's a recent patch that came out for PC version for Ana that allows her to fire 20% faster and her clip can now hold 10 bullets instead of 8. So firing into a group of people with Lucio just got better

Source: http://www.polygon.com/2016/7/26/12291162/overwatch-patch-ana-buff-mccree-nerf-pc