Healing is still fairly rare in Champions (as of Apr 2022), as are inexpensive readying Events. Combining these two abilities into a single card makes this an efficient and effective card.
The Physical resource requirement is an interesting new mechanic. The way the Requirement rule is written, you must pay a Physical resource, even if you somehow find a way to reduce the cost of a card to 0. With this in mind, What Doesn't Kill Me requires thoughtful deckbuilding. Whether it's Quincarrier, Enhanced Physique, The Power of Protection, or simply including a lot of cards with Physical/Wild resources, you want to be able to play this when it comes up.
The only other Protection card similar to this is Momentum Shift. For the same cost, you get two healing and deal two damage and don't have to fuss with the Physical resource requirement. This is a consideration to keep in mind! In exchange for dealing with the restriction you get flexibility to either Thwart, Attack, flip and Recover, or Exhaust to deal with any number of Scenario-specific challenges that require you to exhaust your Hero to deal with it.
Heroes that have a natural 3 ATK would especially beneift from What Doesn't Kill Me, such as She-Hulk, Hulk, and Drax.
This card is a solid B: Requires a certain amount of building around, but is ultimately a solid utility card that provides flexibility and endurance.