The user fragmentation is going to make it hard for communities to reach critical mass.
Explain Like I'm Five is a subreddit where niche experts come out of the woodwork to make the sub phenomenal. However, that doesn't work if those users are split over 100 different subs named the same thing on different servers.
I think the subs/communities need to automatically mirror or aggregate, but I'm not sure if that's practical with this platform topology.