When I was working in Hong Kong and had to go over to China weekly to visit our Chinese suppliers the company insisted I go with a company driver because of the chance of abduction. They wouldn't even let me at least get to the border crossing. The driver was also effectively my personal security which for a no-one like me was almost comedic.
Yet at the weekend I could and frequently did go over on my work visa to the same general area and others by public transport.
And I never faced even the slightest difficulty, in fact the exact opposite- all I ever found were kind and generous folk. Those weekend trips (by bus, train and boat) so enriched my time in HK. In fact the only frightening bit was some of the leaky hydrofoils, especially in rough conditions
I went to Colombia twice at the height of the drug violence, and my shipping company insisted on putting us on tours with armed guards on the bus. Yet from what I saw that felt like prudent overkill.
BART in San Francisco is also reported, in some quarters, to now have the same alleged problems as the LA metro and Seattle and it seems most other transit systems in the US.
Having been one of my special case studies at school in the UK I sure ain't giving up on BART that easily. That's the US system I have used the most.
I get the feeling, and I'm sorry to say it, that many Americans talk up perceived issues with mass transit to justify being wedded to the automobile. Blind to the problems that in itself causes.
I heed your warnings, and will be careful and mindful but bluntly if I arrived in LA, SFO or SEA (or any other American city with mass transit) tomorrow will continue to use it.
Yesterday I was talking with a forum member about why Dallas Love Field airport website tells you absolutely nothing (doesn't give any even vague hints) about public transit from the airport- yet there is a very good bus/metro link.
Again there the answer, I can only presume, is that people are just fixated on their car.