I think people in this thread (and elsewhere) are using "unsafe" to mean "I feel uncomfortable" rather than "there's a serious chance that harm will come to me".

Law enforcement in liberal cities might overlook public urination or petty theft by the mentally ill, but they come down hard on violent crime. Truly violent people are not allowed to roam free on public transit. There's definitely some weird people though, and our society is segregated enough that most rich people probably can't tell the difference between a violent weirdo and a harmless one.

And don't forget, you're more likely to encounter a road-rager threatening you than some person on a bus.