I do think that learning healer on a savage fight is likely the most difficult. I heard M3S is also a fight designed to test healers a bit.

The "Healing being easy" was more towards casual content. where you don't need to heal much even in big AOE situations due to serval different issues stacked on each other (no its not just tank sustain that only plays a part in it).

What people are generally saying/asking for when saying "healing is too easy" is that in all forms of content they should feel impactful and like actual "healers". Instead of only the most difficult kinds of content.