No. Grievous wounds is toxic. Can confirm, please remove from game.
Yes, I DO main Soraka.
In all seriousness, I see the frustration, but it seems odd to me. Healing reduction is essentially the equivalent of damage reduction. Imagine if there was an item that made people on the enemy team do 50% less magic damage to targets below 40% health - not to you, but in general. Of course that would be bullshit. Especially if there were multiple versions that stacked additively, so it dropped to 0 damage.
Why is grievous wounds essentially free, seeing as Morellonomicon is gold-efficient without it? On a champion ability, it almost makes sense... again, if it were 15% like Urgot, or a very short duration. It sorta feels like "This item/ability is pretty good, so you can build it whenever. And if the enemy team has Soraka, well FUCK HER. You counter her for free. Lucky you!"
I suppose it's not so much that I mind grievous wounds, but it sucks that they don't give anything up for it. If they actually were somewhat weaker or the item was more expensive, I could at least think "oh, well he built against me - at least he didn't build against Draven then." Naw, he just needed the cdr, mp5, and ap. He may not even have thought about the grievous wounds.
In practice, it isn't so bad - Morellonomicon doesn't work unless they're low, and even if they get them there my ult can remove it and heal them back above that point. Katarina I have to silence, since if she has Morellonomicon it will apply 100% healing reduction every .25 seconds (though raka's ult will still heal, since it removes the grievous wounds simultaneously." Varus and MF are ad, so they only get one version of it, though if the midlaner builds Morellonomicon then I could be in trouble.
Disclaimer: This is 98% me being salty about the counter to my main and 2% me actually thinking some parts are silly.