As someone who's worked with ejabberd I was about to suggest they're in for a headache but it looks like the peak player count for Eve Online is something like 65k users. They might not even have to cluster.
Well, the most difficult part is to understand how the Erlang VM / Environment works.
Once you have set it up, ejabberd just works. You can have massive scale and large uptime. This, ultimately, saves bigger headaches.
That's the promise but in my experience getting mnesia happy and keeping things clustered is huge pain for ops.
Well, in practice, it is more than a promise. You are not forced to use Mnesia. You can use other backends to simplify the ops. Yes, it is as simple and make managing the platform a breeze. You need to be as stateless as possible.