I think you've hit the nail on the head there: "undesirable place" can come from a lot of reasons. Sure, I think SF has become more undesirable due to the increase in visible homeless population (visibility most driven by the mentally ill and addicted, as well as the tent encampments), as well has rampant visible drug use.

But I think the belief/feelings that this translates to much worse crime is just not correct. I can sympathize and agree with the idea that homeless and drug problems makes SF less desirable, but I don't think it moves the needle that much on safety and crime.