I was wondering, more, has there been any actual research done on this question, or is this just speculation based on personal anecdote? Are we comparing actual "married" poly groups, or are we comparing monogamous marriage to polyamorous dating?

I've never seen anything beyond personal opinion and armchair philosophy that suggests that mature polyamorous relationships are less stable than mature monogamous ones. The bias mostly seems to be from observing people who are new to polyamory (where the proper comparison would be with people new to monogamy - a group that mostly consists of teenagers and an incredible amount of drama :))