At the time of this writing, I have accounts on two servers. One on the big server, and one on a tiny server.
Obviously, the gargantuan server’s biggest issue is performance. That will probably improve with time, but with its size comes some noticeable benefits, which I will touch on shortly.
The tiny server, which I actually joined first, is blazing fast, but I’ve run into constant issues trying to find communities and posts that the bigger server can find no problem. Initiating a federation request is not intuitive at all, and your average user is going to wonder why the hell so much stuff isn’t showing up when they click All on a smaller server.
I ended up manually copying my subscription list from the gargantuan server to the tiny one. It was quite a chore, even though it got better in 0.18. Having to retry a search several times or manually input the URL and reload the page several times until the server can find the community on the remote server is not something the average user is going to want to deal with, so they’ll end up on the huge servers that already know about the communities on the other large servers, if they don’t give up.
Hopefully this gets better, but that’s my best guess as to why everybody ended up on the gargantuan servers.