From an average outsider perspective is hard to know what is “the big one” in a federated system. And i think links online are what break federated systems. If you get linked to some content from another instance than your own. Then you’ve got do deal with the url bar yourself in a specific way that is never well documented (only passed around via word of mouth) so you can log in and interact with said content. A baked in “instance switch” at the top of such services that would redirect you to the same content on another instance would be the best solution but I’ve not seen anything like that yet.