There's some mechanics that need to be sorted out with federation. But I say this while not offering any help to Lemmy, so I'm just ranting and not criticising where the project is right now. They only have so many devs working on it and the founders are doing overtime to bring new features in.
I'm banned from lemmy world apparently which means that lemmy world users who interact on lemmygrad or any other instance will not see my comments, as they won't be federated (copied over to lemmy world's database essentially). I can see some problems with that because while I get the principle, it effectively allows instance administrators to determine who can read whom. If you make a friend on another instance too and they get banned or their instance defederated, you'll lose all contact with them.
Perhaps even worse is that lemmy.world users who come to lemmygrad won't be able to read my comments, which is just weird because this is my instance. I effectively cannot interact with them even on my home instance! Being an admin too makes it more complicated, like if I posted a pinned message to welcome users from X instance and I'm banned on that instance, they effectively wouldn't see it. You also don't get a notif when you're banned or unbanned, so you would never know.
But I'm sure these will be improved in due time.