You're also going to have to deal with cases where the creator of a sub-community fails to do their job as a moderator (or actively works against it). In the limiting case, if you get a DMCA takedown notice, get a GDPR notice, get a C&D, discover illicit materials of other types, discover that people are using your site to plan crimes/attacks/etc, or any number of other things, you're still responsible for that.