I don't think the /r/cities is comparable to nextdoor. In the last nextdoor community I was in people were ready to lynch a guy who was schizophrenic because they decided he was dangerous. They started a pedophile manhunt for an old man who was literally looking for his lost dog. The /r/cities all seem to be removed enough from the immediate local that they didn't feed this sort of vitriol. But I suppose ymmv.