For what it's worth - on the microblogging (twitter-like) part of the fediverse, we've had pretty great results federating with lots of instances, but defederating from the ones that have problems with hate speech, spam, etc. It's led to a much more positive vibe than Twitter etc because people aren't so afraid of some random person misconstruing something they said and jumping down their throat about it, and there's no algorithm pushing rage "engagement" to the top.
One of the problems we do have in microblogging land, is the main mastodon developer has tried to direct new users to his own instance, making it by far the biggest mastodon instance, and then did a bad job controlling spam - so the other instances were left having to chose between cutting off a huge number of real users, or being inundated with spam. Having all the kbin and lemmy instances federate with each other, except where there are issues, can make the network overall more resilient against any one instance being mismanaged or invaded by spammers and trolls. The users on that instance will have to move, but there will be plenty of similar-sized instances ready to welcome them, and if the magazines are also spread out, they can keep posting to the same ones.
(I admit I also have a selfish interest here - I'm posting this from fedia,io, a different kbin instance. I joined through a different instance than the main one to help nudge things toward decentralization.)