Yuumi really needs to have additional counterplay when attached to a champion with its own healing

Late game Sylas, Singed, etc. are all terrible offenders; anyone with high survivability, damage, and a form of self-healing or regen. When Yuumi attaches to one of them, they become borderline invincible and both counter play and skill get thrown out the window.

Grievous Wounds does nothing, because the amount of healing is so excessive.

Possible fixes could be to make multiple forms of Grievous Wound stack with diminishing return, or just lower Yuumi's healing on champions that have recently used healing spells. This is the exact reason that the summoner spell Heal works the way it does.