If you have a group of people who are not interested in the entire forum but only their category, mailing list mode is even worse. But I know/hope that [mailing list mode per category]
(Apply "mailing list mode" per category) might eventually be supported, so this may become a non-issue in the future.
I’m also interested in negative examples. So those cases you’ve seen fail, they tried to build a bridge by forwarding certain emails to an outside mailinglist?