Nuclear families seem to be empirically less poverty-prone than other family arrangements. And there seem to be higher order effects to the success of entire communities when there are relatively fewer nuclear families in the neighborhood.
Do you mean as opposed to singles or to extended families? In either case, I'd be interested in a source.
https://en.wikipedia.org/wiki/Poverty_in_the_United_States#P...
https://www.ssa.gov/retirementpolicy/fact-sheets/marital-sta...
Marriage is mostly a "capital" redistribution system and with strong public services (ie shared services) there is much less need to focus on the nuclear family as a protection (sharing of ressources) network.
Ex. When a few years back pension laws changed in Japan and housewives got the right to get a pension separately from their husband, the rate of divorce boomed.
There is nothing in the nuclear family that makes in a natural fit to what humans are as social animals.