The grandest issue is that he was supposed to be more in the frontline when they reworked him but they had taken away self cleanse and took away Ancestral on Rehgar in addition.
Auriel and Uther comparatively can still cast Aegis/DShield on themselves. This ended up with Rehgar in the absolute backline again being a healbot and nothing else.
Another contributing factor was Greymane who dominated early 2016, sustained healing didn't do shit but landing an ancestral on him was pretty much a won teamfight.
With Greyman nerfed, Li Ming nerfed, KT nerfed, Jaina nerfed, Valla buffed twice over, Lunara buffed, Zagara buffed,... BW didn't change one bit but went from bottom to top tier healer.
Merryday made a smurf that was basically called Only playing Rehgar and Uther for a year feels bad. No idea how he fit that in so few characters.