Someday, some statistics will be more readily available-as college students and scientists have already begun studying alternative lifestyles.
But it will take years to collect a reasonable amount of information to even start basing educated guesses on it.
At this point, the best information available is probably what works/doesn't work in general for any relationship. Take that and multiply it exponentially (check out galagirls links on polymath tiers for explanation) to identify the risk places in your own relationships.
(for example, if you know one or more of you is weak in interpersonal communication-that's a risk factor).