As far as I can tell, this would subscribe you to a user with the username "fediverse". I've already seen a user and a community both named "cat" on the same instance, so that could be a problem if calckey doesn't differentiate between @[email protected] and [email protected].
Also: How does moderation work in this context? Does a post/comment that gets removed by moderation in one community get removed in the others as well? If moderation can edit titles or text, does that carry over to the other communities?
The issue is that ActivityPub, the protocol used for federation, wasn't designed with reddit-likes in mind. Upvotes are repurposed Mastodon "favourite"s, and boosts are Mastodon's equivalent of "retweets", but downvotes are brand new with the appearance of lemmy/kbin. One could extend ActivityPub to have a new thing for downvotes, but non-lemmy/kbin instances like mastodon servers wouldn't have any use for them.
Also, a not insignificant number of lemmy instances don't have downvotes as a deliberate choice.