One huge shortcoming of the Fediverse as currently implemented is that you have to use a different ID for each app, ie tube.jeena.net/@jeena, toot.jeena.net/@jeena, etc. What if you want to use the Mastodon UI to write a comment on a video? Your comment is going to use a different ID than the one you normally use to interact with videos. Given this, what's the point of federating these different apps with each other?…
Mastodon instances basically _work great_ with each other, and with Friendica and Calckey too, since they have the same Twitter-like structure. Lemmy and Kbin - and all instances thereof - can chat with each other without issue too, since they have the communities + threads structure in common.
But trying to get Mastodon and Lemmy to talk to each other is 1. surprisingly possible, and 2. kind of annoying, since they speak the same protocol but have different structures of communication. And never mind other ActivityPub applications like BookWyrm or WriteFreely or PeerTube.