Once a person from a certain universe arrives, there's some chance that they can trigger a chain reaction and bring in more people from their universe. That's nothing we can control though, and in some cases, it can bring mortal enemies through which isn't exactly conducive to any form of peace on the ship.

Also, there's a lot of universal 'loners' onboard as well, so it's kind of... hm. [Insert vague hand gesture here.] I wouldn't exactly rely on it, is what I'm saying.