Pippo, you must differentiate between a composite chart and a synastry chart. I dont know where you got this:
how many positive composite aspects are needed to make the relationship outcome work, as synastries arent enough and are only temporary when the relationship starts.
.
A Moon-Pluto harmonious aspect alone is certainly not enough to make a relationship work. Moon-Pluto in itself is a difficult combination and invites powerstruggles, even with positive aspects between them. I would most certainly never take this aspect as anything positive. Karmic maybe, but then it can be really difficult karma to get away from.
A synastry chart shows how two people interact with each other based on their individual charts.
A composite chart is a chart of how others see the relationship. A composite can have very nice aspects, but this does not mean that between the two people all is fine and dandy. They just project this "fine and dandyness" towards the outside world whilst in reality it is a Virginia Woolf situation.
Also the other way around is seen. Two people who do not look as if they are very close together, might in fact be very close in their private surroundings.
The composite just shows that they behave a bit distant, dont want to be too demonstrative about their love.
So the Synastry is the real thing. But should the synastry be difficult, then of course the couple could work towards a more harmonious relationship as seen in the composite, (after all, they can behave when in public) but that is all a matter of maturity between them.
So from that point of view synastries could be "short term" as you say which is to be hoped when the synastry is not harmonious. But usually it is far more longterm than you hope for and often never changes. By progression planets in both charts will move into new signs etc. and that could temporarily create more harmonious circumstances, or the other way around of course.
I never use synastries but only look at the individual charts and the progressions. From that you can see everything you need.