And people act outside the bounds of the law. SSN seems like a good natural key candidate, until you discover some of the people your business interacts with are illegal aliens.

Or legal aliens, like tourists. Or foreigners, who aren't even in the country. Or really anyone else who doesn't work in the country, and doesn't have to report taxes.

Ironically, many illegal immigrants actually do report taxes. In which case they use ITIN, which is compatible to SSN format-wise (###-##-####)... but it is not unique over time.

Definitely true, but at a certain point you need to draw a line. If these things were to change, the entire product would have to change, not just what I use as my keys, so I think natural keys are a safe bet in my case.