Karazhan is a perfect example of how to make content challenging without pushing everyone's mistakes onto healer.

Karazhan was one of the most enjoyable experiences i've had as a healer in legion. Bosses had enough of steady damage to keep me on my toes without being too stressful, and i still felt impactful since i could remedy someone's minor mistake or two. At the same time, if someone was failing hard they were going to get instantly killed or getting entire group killed instead of forcing me to struggle helplessly against overwhelming damage that was going to wipe us anyway, like in mythics+. For the first time in a long run, healing in 5man felt like fighting my own battle against boss mechanics while still being able to help others to some extent, instead of being a scapegoat paying for every single mistake made by your group and basically doing nothing when ppl were not making any mistakes.