One of my friends who's in a triad has said she doesn't think that polyamory is consistent with maximum achievement-- intimate relationships with more people simply takes more time and attention than being in a two-person relationship.

I was thinking about that, and realizing that none of the people in the triad are extremely ambitious.

Polyamory might well be consistent with achievement if the group includes ambitious people, and might be better than monogamy if the number of people who want to be in support roles are more than half the group.