There are lots of phpBB forums, and their content is many times invaluable. The application itself though, is completely terrible. It's not that the information isn't there, it's the finding that sucks. For example: basic things like having the search index work properly with unicode. The chances that the php, db and app settings are all properly set up for searching for "ö" are slim to none.

I don't agree with all the design decisions of Discourse, but it was (seemingly) designed to be a straightforward migration from phpBB and other similar forums (flat ones). Just having a proper search alone is worth the migration.