When considering this idea, it's worth keeping in mind the differences in what healing actually does in Overwatch and LoL. In Overwatch, healing is a mechanic you expect a team to have, it serves as teamwide sustain, and it's incredibly potent. You can afford to create downtime on what is essentially a team resource in order to create openings to exploit. You don't really fuck any particular hero by denying healing for a short time. League doesn't work the same way. Healing is primarily a self-survival mechanism (only Soraka offers good in-fight healing), and it's not really stronger than other survival mechanisms. It's just one option out of many, and we don't really have any need to completely destroy it when it's not outperforming other options.
Would things be more consistent if Grievous Wounds completely denied healing,
What would be consistent is that you'd have to balance characters around the expectation that they'd never have healing available in any teamfight scenario. You'd have to rebalance healers around being completely functional without having access to healing.
If you're going to have a purchasable effect that could completely block out a character's mechanic, then either there have to be a lot of potential ways to force that effects to be burned (such as how QSS invalidates CC but there are so many CC effects on a team that you can force it to be used on one effect instead of another), or the effect has to have some huge gate put on it (and a 2-3 second duration is not a significant hindrance). If neither of those are true, then you basically have to rebalance healers around the expectation that they just don't have the heal in their kit anymore.
Grievous Wounds has already been responsible in the past for creating huge balance problems where healers either had to be strong enough to make do with their reduced healing (leaving them annoyingly unkillable whenever Grievous Wounds wasn't an option), or they were balanced without Grievous Wounds. The middle ground between these points is virtually nonexistent.
If we do anything with Grievous Wounds, I'd advocate for removing it and nerfing healing to compensate. That would produce consistent results. Barring that, we should work much harder to ensure that grievous wounds is NOT avalivable for the duration of an entire teamfight. Slightly strengthening it and attaching it to the active effect of an item would improve its design significantly, and would create openings to reblance heals if necessary (something like, your next spell gives 60% grievous wounds for 3 seconds). Having it as accessible as it forces the stat to be weak, because having healers denied a core mechanic of their kits over something so binary and out of their control is terrible design.
