My main MMOs were Grand Chase and Ragnarok Online. GC healing was nearly non-existent since you could dodge boss mechanics by animation-locking yourself out of it with abilities.

Ragnarok Online on the other hand was extremely hard to heal in every aspect. No aggro table whatsoever, only AI commands - wich means MVPs could suddenly lock target on you and screw your life. At least 8 buffs to keep track in a party of 12 people with no party UI whatsoever. PVP was even harder because no way in hell you could see clearly where your guild members were. The only downside in that game was the lack of healing options. A full support would spam Heal and put Sanctuary and keep track of buffs. They made it better with more options for healing with RO renewal, but I quit just before that.