Yeah, but it's also impossible for mercy to get decent positioning due to the short range of the beam. You're always in danger and against a decent team, the first target. To make use of your ult you have to usually hide, leaving your team in a 5v6 situation, again anything but ideal. Also healing with an Ana alone is still strong enough to mostly counteract the effects of rez, because she can still get her team to full health very quickly, usually the time from the end of the fight to the rez is enough.
Also yeah, Ana's nerf made a totally OP ability into a still OP ability. Sure you can't burst heal as crazily anymore (325 in ~2 seconds before, now ~280 in the same time frame), but anti-heal is still just as lethal (and does a nice chunk of damage as well).
Mercy works great against disorganized teams though. If they don't focus her and she can remain in the fight until the rez is necessary, she is an asset. It works even better if the enemy team doesn't have Ana, since no one can remove her only utility from the game then. Currently in organized play, the only reason to pick mercy for me is to support a really good pharah, though depending on the map/situation I also use Ana for that.