Lots of people have visited SF as well as more densely populated places both in the US and elsewhere.
So have I - and downtown SF is worse (in terms of littering, human excrement, aggressive panhandling, street crime) than the problematic parts of NYC, DC, Philly and Baltimore, worse than densely populated capitals in Latin America or East Asia or Eastern Europe. It's better than Indian cities but that is a very low bar for one of the world's richest cities.
I sort of doubt that you've spent much time in the most problematic parts of DC/Philly/Baltimore. Their peers are Chicago, New Orleans, and St. Louis. Not remotely SF.
So have I - and downtown SF is worse (in terms of littering, human excrement, aggressive panhandling, street crime) than the problematic parts of NYC, DC, Philly and Baltimore, worse than densely populated capitals in Latin America or East Asia or Eastern Europe. It's better than Indian cities but that is a very low bar for one of the world's richest cities.