I think vast majority of the community agrees that healing spells are too strong on single model units and sometimes downright abusive, as well as low model cavalry that benefit from it disproportionately. It is especially obvious on Vampire Counts and Vampire Coast but also to a lesser degree with High Elves and Empire.
However I don't think anyone has expressed any problem with healing on lower tier troops, and indeed it seems to be an essential part of army compositions of Vampires, High Elves and Empire. The problem here is then not healing in general but healing specifically on high value units that are able to soak damage. An easy, general solution that affects both Vampires and regular healing and properly address the problem of healing on low model count units without making it completely useless on lower tier troops is I think changing of how the heal mechanic works on the ground. This could also affect Tomb Kings and help their battle ability a bit as it is currently a bit underwhelming without making their single model monsters too powerful.
For this I would suggest all healing spells should work on a per model basis instead of a total spread over models. That is, healing should heal every model for a specific and much lower amount that would then lead to a total heal over the unit. This would mean that healing would be more effective on units with more models while being almost completely ineffectual on single model units. Now you might say then what will the single model units do? Well for that we can keep spells like apotheosis, regrowth, immortal will (vampire heroes) and blood chalice (Isabella) amongst others which would heal only single models but for a great amount, making them effective on single model units but ineffectual on multiple models. It would also make passives like the hunger more valuable.
This way healing can be balanced individually and exclusively on multiple models and single model units without making it completely underwhelming on one or another. It would also not kill the character of Vampire Counts and High Elves especially.