Well, unless it's changed, the big problem is that it seems the first person to sign up is the moderator/leader/whatever. And those are the people with nothing better to do, who subdivide to gain power over some amount of houses. Basically, like the people who try to form HOAs. Most complaints about Nextdoor would be fixed with reasonable moderation.
Also: I think a lot of people who complain about Nextdoor need to reflect on where they chose to live. My little city in KY was perfectly fine. People offering help, lost/found animals, events...standard stuff.