if you are seriously trying to kill mundo you are going to need more than a few pokes here and there. Anything more than that will get through this item's defense as it will only be blocking something like 5-10 damage per second. that will be a nerf against fully committing champs.
On the otherhand it will also mean that a champ like mundo who only hits you every now and then will actually be less oppressive
Akali will have the same issue, this item will actually stop her poke damage, same with katarina.
Zed will also lose out to this item against anyone who's using it.
zed will not become unkillable, if you commit to killing zed then this item will actually prevent less damage than normal dorans blade.
The item I designed actually stops noncommittal damage not all ins. Against an all in attack it is effectively a small boost to hp regen that wont actually recover your hp.
Unlike hexdrinker... this item does not care what kind of damage it is, it wont block one ability and not another, and it doesn't give zed ANY damage and will not build into any kind of damage ever. It might help him against ranged but it will also help ranged against him if he builds it.