I've been on the fediverse since 2017. Anyone who's dealt with running an instance knows how much of a pain in the ass dealing with huge monolithic instances is.

Recently on Mastodon for example, Mastodon.social has had huge spam waves of bots creating accounts on it then randomly sending replies with spam links to anyone they can find. And of course because Mastodon.Social is a huge instance with not enough moderators, people on outside instances can't really do anything except whack-a-mole with the constantly new accounts since the "flagship instance" has open registration. At one point the instance I use now had to suspend mastodon.social temporarily to make the spam wave stop, which of course screwed up everyone's follows.

The best part of Mastodon is the federated nature of the network, which gets completely screwed up when you have mountains of people on a handful of "too-big-to-suspend" instances rather than have people spread out across hundreds-thousands of smaller spaces.