That invitation might have come from me. Anyway, I checked the link you mentioned, and was quite surprised at the level of vituperation.
Conversation among Nextdoor's Brixton users is nothing like that. So far people have been utterly civil and co-operative, and discussion is about the usual mundane neighbourly stuff - lost cats, yoga lessons, borrowing gardening equipment, broadband problems, cleaners, babysitters, etc. I have used it to spread word and solicit opinions about the Lambeth Air Quality Action Plan meetings. I think that it's quite a useful tool for building a sense of community.
It's different from and more immediately useful than U75, in that there are no tedious and monomaniacal multi-page flame wars about which poster holds the more virtuous ideological beliefs. None of the Nextdoor users - so far - feel compelled to launch jihad against others for incorrect thinking.
This is an informative article:
www.dailynews.com/20170307/how-social-network-nextdoor-is-becoming-a-new-way-to-neighbor
I suggest you and anybody else who is interested sign up and see how it goes.