you had better start imagining pretty hard and consider every possible unexpected event of that order of improbability, including black swans

With QS you must guard yourself against all local Everett branches. Those branches could conceivably contain black swans, like a few electrons tunneling out of a circuit preventing a CPU from performing correctly. Even that is a 1:1,000,000,000 or more event. But they will not contain something macroscopic.

If I look around and notice no one nearby, I might say "I am only 99% confident that there isn't anyone near." If I then sample all local branches (with a device that has a 1:1,000,000 fail rate), killing myself in those branches that no one appears, what is the probability that I will find myself in a branch with another person nearby? I would say about 1%. The presence or absence of another person should behave classically for the small numbers we are talking about. Quantum probabilities are different than my own Bayesian probabilities.

In short, while some failure modes will become more common, others will not.