From what I’ve heard, only communities from other instances that users in your instance are subscribed to will show up. This is because (as far as I know) the instances only request data from other instances of what users have requested, and not a constant stream of updates that no one has a need for.

Yep, using it right now. I honestly don’t even miss the other site anymore, though Apollo (and AlienBlue) will always have a place in my heart.