I think fears about the homeless robbing you are massively overblown. I’ve been panhandled hundreds of times, and the worst response I’ve ever gotten is a dirty look.
I also think that the people shitting on public, etc. are more likely to be mentally ill than addicted (although they could be both). The reason is that Drugs Are Really Expensive. Also, I’m not aware of any addiction that causes you to shit in public, but it’s easy to find mental illnesses that would.