@Brendanjones
Activity pub can do private conversations assuming both platforms handle that correctly. The issue is that you have to trust the server admins of all parties involved, since it's not end to end encrypted.
As for the limits, I think it's mainly that the base protocol is limited to ensure it can be adapted to anything, even if imperfectly. Many mastodon clients don't play peertube videos properly, and it's a challenge to properly follow lemmy posts on other platforms.
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