My experience with Nextdoor and local Facebook groups has been mostly negative - it's not particularly civil at all.
Although I think some of that stems from the Eternal September factor - some people just do not know how to effectively communicate online. It's wild out there.
We've known that since the halcyon days of USENET when everyone put their real name and university emails in their signatures. We also know it because it doesn't work on Facebook, and it didn't work when Google decided they needed to do a Facebook and they forced everyone's G+ account to merge with Youtube. People who feel entitled to their opinion and lack empathy will absolutely, with their whole chest, be an asshole and sign their John Hancock to it.
The truth is, you can't engineer civility. Anonymity? Look at 4chan. Make people pay? Now the assholes believe they've purchased the privilege to do whatever they like. Even Hacker News, a platform moderated to within an inch of its life, with both real-name and pseudonymous accounts, only tends to be civil within a small window of circumstance. Everywhere else it's as likely as not to be a dumpster fire.
Although I think some of that stems from the Eternal September factor - some people just do not know how to effectively communicate online. It's wild out there.