nowadays, it seems like anything unmoderated starts going right. it used to be the opposite.
mildly OT - the only thing they have in common is that they are infiltrated by bots which just serve to amplify the extreme discourse. there are some interesting bot-bait threads on 4chan where people communicate text via obfuscated text images, but bait bots with text replies (i.e. "hillary bad pizzagate"). the bot replies then become obvious because they reply to the bait text which is off-subject, but the real people reply to the text images with obfuscated text images. the bots were right wing extremist and comprised maybe 60% of the replies.
i saw someone had an interesting idea to make a "deep fried" image board or plugin, where text replies are posted as obfuscated images which are unreadable by bots, so people can more easily distinguish bot replies from human. i imagine discourse would improve considerably.