The problem with grievous wounds even existing is that it allows healing to be so strong that I have to alter my build drastically to counter it.
Some comps don't need armor pen. Entire enemy team building squishy? More crit and AS please! oh wait they have swain? booo...
Entire enemy team stacking mad HP and armor? Oh boy, giant slayer passive is great! Oh wait, theres a vlad, I have to get the other one and do less damage to the other 4 enemy champions.
I understand how to counter it. I couldn't have gotten to play without learning how to adapt my builds to the situation. The problem comes when there is only one option for dealing with a situation, and that option removes better options. And before you disagree, look what happened with zed and QSS. Way back when, before mercurial scimitar was actually good, zed had to be nerfed pretty hard because ADC's were forced to build a magic resist item to deal with a physical damage assassin, and that item wasn't even good. Fast forward a bit and mercurial scimitar is buffed and (I can't remember if zed got buffed or if the meta shifted to favor him) zed started to come back. Key word though is "started" to come back. People quickly began building the new and improved merc scimitar and zed was essentially useless. And finally, they nerfed QSS to not remove zeds mark, and now zed is back.
The problem with 1 item fixes is that they are either too strong and nullify the champions they were designed for, or they are too weak except when dealing with that one specific champion, but because that champion exists, you have to forgo the superior option. Right now Mortal Reminder is in the second camp. You're unable to get giant slayer passive (which is FAR better) because vlad and swain simply exist. If riot buffs mortal reminder to not suck so much we'll never see vlad and swain again.
The real underlying problem is the AOE healing these champs do, even during stasis. Zhonyas should disable or reduce self healing, and AOE healing should be massively reduced after the first target.