I’m doing what Stephen Ellis suggested; I’m working on a rather large entry for the wiki that will answer questions of this nature. “Can I make an open-world online game in Unreal Engine” and “Can I make an MMO in Unreal Engine?” Even though I agree a bit with what The Britain said above, I think it’s nice to have a link to answer questions like this with.
I will post a link here when it’s done, and then I encourage people to edit and add to it.
This first article will be about “Can I?” and not the technical details of “How?” - that might come later.
EDIT: Here’s the link: A new, community-hosted Unreal Engine Wiki - Announcements and Releases - Unreal Engine Forums