It wasn't just unfair and unfun because the boss actually was designed to also make fun of you after your get hit. The add just grabs onto you and you can't do anything for ages while it carefully puts that AoE over your head, taking its sweet sweet time to hit you with it before letting you go (alive or dead). Losing complete control and agency of one's character is generally one of the worst thing you could design in a game, and it feels even worse when the game actually makes fun of you while doing it.
I feel like this stuff would have been infuriating either way, but people would have been a lot less mad about it without that middlefinger you get once you fail the mechanic.