The federation with Mastodon is mostly one-way: We can’t see or comment on Mastodon posts, but Mastodon users can see and comment on Lemmy posts.
Mastodon’s like Twitter… its posts wouldn’t fit in the Lemmy UI well. Though I hear kbin works well with both Mastodon-style and Lemmy-style posts.
… it’s not a downside of the protocol, it’s just a literal impossibility. Once someone’s downloaded something, you can’t do a thing to take it back.