To make an analogy, I will use DnD 5e. Wait a moment, you'll get what I mean in a second.
Let's say you want to play a survivalist, a character super good at tracking down things and finding ressources in an hostile environnement, lost in the jungle etc. You pick a druid or a ranger, get a good rank in survival and nature, take spells and abilities that makes it easier to navigate and find your way.

But in actual sessions, it just means that this whole part of the adventure is trivialized and made irrelevant by your character. You made this character because you wanted to experience exploration and survival, but their abilities make it so easy that it'll likely be ignored completely. Paradoxically, if you enjoy that survival, exploration, getting lost and finding food etc aspect you are better off playing a character that is NOT built to do this thing.

Healers are kind of in the same situation. I assume most healer mains play healer because they enjoy healing and managing the survival of their party. But if the fights are too easy/their tool too powerful, they won't have much to do or to enjoy.
Imagine if a DPS killed all the ennemies a third of the way into their rotation. As if everything was like floor 55 of palace of the dead. That's what healers are complaining about.