I think the hardest thing to overcome will be community duplication across instances. When searching for my old subreddit subscriptions' parallels, I had to dive into the duplicated communities a few times to see which one had more posts/were more active.

Of course, the gentlemanly thing to do when creating a community would be to check first, but that's obviously not happening all the time. Then there's what to do if one gets created; should the instance admins get reports and yeet them if it's determined to be a copy?