Things do go wrong but usually it's by the players you'd expect (sprouts and returners). And often you can just patch up their mistake with an oGCD heal or Esuna unless they are really not using their head to learn and observe their surroundings (I had a few such cases recently).
But there are healers that, instead of just using an oGCD heal will just keep 100% uptime on adlo "in case" people take damage so pre-empt their mistakes. Nobody notices this, because it's extremely, extremely common and succeeds at preventing wipes, but I do see it a lot, just like with White Mages spamming cure.
I can see the argument of preventing mistakes from killing people by applying shields, but I don't personally pre-empt mistakes - only unavoidable damage (unless there's something like an aoe marker being overlapped on me and I can't move away).
So the argument then is: is a god-tier healer one that spams DPS and only prevents unavoidable damage? Is a god-tier healer one that prevents all damage by keeping shields up 100% of the time?
The answer is it doesn't matter because you probably don't even notice in most runs unless the DPS of the other roles is extraordinarily low.
For sure, like 50-75% of the time healers don't even Esuna...