It is cool, but lately there has been a bit too much hassle with it - my instance (latest versions mind you) is federating like shit.
There are often posts with wildly different upvote and comment counters, comments not loading at all even after days and so on.
I tried looking into it, both in issues on GH and Matrix room but no-one has time:/
Currently this process is rather painful as there is no way to look at all local communities of remote instance thru your instance and subscribe to them directly.
Instead there are two ways to get a remote community subscribed locally.
You can also do the URL-in-search-bar trick with posts and comments to forcefully load/sync them locally.
Keep in mind that instances will push new community events (upvotes, comments, posts) only when there is at least on subscriber, so in the case you fetch the remote community locally but do not subscribe, it will not be be actively synced and only few posts will be visible (with possibly wrong karma/comment counters and so on).