Forums are great, but they are still susceptible to a lesser version of the difficulties faced by broader social media.
Specifically, a massively diverse user base all coming together in one giant, free-for-all discussion area. A diverseness and freedom of social interaction which, under OG "real physical world" conditions, would only ever have occurred in rare, carefully choreographed social occasions.
It results in extra work, on several fronts. Moderation (including self-moderation). Conflicts arising from different perspectives (e.g. socioeconomic). Trivial misunderstandings. Tediously-long explanations, in attempts to get your point across to all - or to avoid accidentally offending.
But the payoff is a great broadness of information, perspectives and interactions.