The spell specifically states that the damage cannot be reduced by any means. That includes spells cast or passive resistances (such as a vampire, protection from certain damage types, etc.). Even if it could, it would also reduce the amount of healing, because the heal is based on the damage that you take. Jeremy Crawford already ruled that it is not a healing spell, and thus not subject to any healing modifiers such as the life cleric feature or Beacon of Hope. Life Transference is pretty locked down and exploit-proof imho.

The spell specifically states that the damage cannot be reduced by any means. That includes spells cast or passive resistances (such as a vampire, protection from certain damage types, etc.). Even if it could, it would also reduce the amount of healing, because the heal is based on the damage that you take. Jeremy Crawford already ruled that it is not a healing spell, and thus not subject to any healing modifiers such as the life cleric feature or Beacon of Hope. I'm pretty sure that's because the magic behind it is necrotic, just like False Life is not a healing spell because it gives you temporary hit points rather than healing your hp lost to damage. Life Transference is pretty locked down and exploit-proof imho.

P.S., I think the only way it would be OP would be if it granted temporary HP after max HP is filled.