On the surface, Quantum Immortality is an attractive thought.

Under the many-worlds interpretation of quantum mechanics, every chance event leads to the creation of multiple parallel universes. When you roll the die, it doesn't come up 5. It comes up six different universes, and your personal thread of causation just happens to be looking backwards from the perspective of the '5' branch.

As a corollary, if you are ever in a life-threatening situation and there is a possibility you might survive, in at least one universe, you will.

Sheldon: Penny, while I subscribe to the "Many Worlds" theory which posits the existence of an infinite number of Sheldons in an infinite number of universes, I assure you that in none of them am I dancing.

Penny: Are you fun in any of them?

Sheldon: The math would suggest that in a few of them I'm a clown made of candy, but I don't dance.

– Big Bang Theory: S3. Ep3. The Gothowitz Deviation

This leads to the superficially awesome thought that, at least subjectively, you can't die. Your subjective consciousness will always be looking back through that path of causality in which you survived.

This would be great if existence was a binary state between being dead, and being perfectly healthy and able.

Except It's far more likely that as time increases, the number of universes in which you are not a brain in a jar, screaming your insanity into an eternity of nothingness, approaches zero.