Nothing prevents it totally. It could 100% be just as bad as Facebook. But only those servers owned by those propaganda pushers. It is harder to take over the whole and push your agenda to all users, when they can simply block your instance and let you stew in your hate. And while I do think there will always be nationalistic fuckwards, I think the big number now and in the past are only possible due to massive propaganda. If that is not possible, people steer more to “normal” stuff. Because most people are not majorally negatively affected by the status quo.