I'm not arguing with you on AOE ults doing good work, they work great, but you can't have them up nearly as often as nanoboost. My experience as Ana has been that it is not hard at all to keep the nanoboosted target alive as long as they keep corners in mind. If my main nano target is being stupid and dashing behind cover to die, I'll simply use nano at times when it's unlikely the target can manage it. I've played both with randoms with large skill gaps and with competitive 6-stacks, and while I have to treat engagements very differently depending on the level of communication and coordination, I've never had issues keeping nanoboosted targets alive once I see how those players use it and I'm far from being great at aiming. Somebody who's not used to playing Ana and isn't used to the level of situational awareness needed to properly support a team will probably not be able to reliably save targets with her though.
If I can communicate with my preferred nano target, I'll tell them to forget dodging most things and self-preservation in favor of dealing damage as effectively as possible. As backwards as this sounds, it makes them easier to heal and they survive better and deal more damage in the end. With around 60 hours of support under my belt I've got a good enough understanding of how most players/heroes react and move and can reliably hit them as Ana when they are mobile. I think most support mains would have also gotten used to such things and can do this easily.
The issue is that support mains are fairly rare and most people who heal usually end up doing so because nobody else picked healer and end up doing a terrible job. So I'll agree with you that in general, you won't see Anas consistently hit moving nanoboosted targets(or anyone for that matter).
Edit: Damn I write too much, sorry.
TLDR: You're right that aoe ults work well but you can't have them up as often as nanoboost. You're also right that most Anas you run into won't be able to reliably hit anyone, but any support main worth his salt will have no trouble. Good support players are just rare.