ueharaa,The one who's got this down to a science, in my opinion, is mir. So, much of my working knowledge these days has been supplemented by her expertise.
The progressed natal is how far we've come, where we are, and what we're doing. The progressed MOON, for example, is our emotional development, with the pSUN changing more slowly, as our identity shifts over time. New aspects are made as the natal ones fade, or become stronger.
Progressed to progressed synastry is akin to finding a big box of costumes with your significant other (or, whomever possesses the other chart). Say their pMOON is going through ARIES, and yours is LIBRA. They're likely to grab the viking hat and a wooden sword. You might grab a judge's robes -- and a box of chocolates. He's going to take a class in stage combat, whereas you might opt to learn mediation.
If you're both natal MOON in AQUARIUS, you'll likely enjoy exploring these different facets of yourself in a theoretical, intellectual capacity. But, if one is a lunar Piscean, and the other, MOON in LEO, the latter is more likely to approach it theatrically with a means to gain greater attention, or move up in the ranks; the MOON in PISCES may be more tempted to go on a pilgrimage, looking at the spiritual angle, or thinking about donating the costumes once they're done with them.
So, we've always got to take into account the natal in relationship to the progressed placements.
I've never been a fan of a lack of natal synastry in any capacity. To me, progressed synastry is the switch that can get flipped, which must be attached to some mechanism present in the natal synastry. No mechanism -- what does the switch do when flipped? Not much -- if anything.
I've discovered that, once there's natal synastry in play, that pVENUS to nSUN, or nVENUS to pSUN aspects are fabulous for getting the fire ignited.
Will it last? I look to the natal.
If you've got progressed aspects playing off of natal aspects, that's a winning combination.
I've been told that pVENUS-nSUN / pSUN-nVENUS square can certainly throw a monkey-wrench into the whole thing. Rather than 'deal breaker', however, it depends upon the natal synastry. If there's nothing there, AND a hard progressed to natal aspect ... oooooh. That's rough. Will it doom the entire possibility of a relationship? I don't believe in such permanent pronouncements -- but -- that's tough to overcome. However, if you've got natal synastry WITH a progressed-to-natal hard aspect, that will pass -- eventually -- in which case, the situation may certainly change.