There are degrees to word of mouth, though. Stumbling on, say, HN comment from a year ago, that said one can find high-quality Star Trek discussions on /r/DaystromInstitute, is a form of "word of mouth". Being invited to a community personally by existing community member, who themselves were invited by an existing member, ... is a different kind of "word of mouth".

Like everywhere else in the "cozy web" reality, it sucks to be an outsider looking to learn something or start to participate somewhere. With increasing number of hobby groups moving to Discord - or worse, Whatsapp groups - there's no way to observe from the distance or "dip your toes". Instead, you're asked to commit time, effort and/or reputation from the get-go, even before you know if the thing it's worth it.

This very mechanism keeps me away from any communities I'd happily participate in if they were open discussion boards.