I've heard quite a number of stories. I don't like San Francisco, it's slummy and unpleasant. Sure there's nice architecture and so forth but so do most cities.
I haven't been harassed by homeless peeps. Occasionally they've come close then quickly retract or whimper. I've walked at night around "dangerous" parts but compared to Nairobi, American thugs aren't dangerous.
I think who's "dangerous" is that creepy white guy who's probably a serial killer, pedo, morally bankrupt sadistic psychopath.