I think you and your parent comment may be using two different definitions of "nuclear" though. One definition is the traditional mom-dad-2.3 kids arrangement, but a broader and equally valid definition can include polygamy, gay parents with a surrogate, lesbian parents with a sperm donor, couples where the mom and dad have split and started new families but still have kids in common, and a million other combinations. The most important positive factor seems to be stability, which is easier to maintain when your peer group is accepting of whatever weird arrangement you have. That acceptance is a strengthening factor for the traditional nuclear family as well as the non-traditional combinations.
"Nuclear" should mean that newly married couples form a new household rather than integrating into the household of the extended family.
Why the nuclear rather than the extended family would be considered important for social stability is anyone's guess. We live in an incredibly atomized society that values personal freedom and autonomy over interdependence and social connection, but that doesn't mean "stability".
