From the Perspective of Longevity:
Many factors influence the longevity of an oil painting, including: the artist’s choice of support, size, ground, pigments, and binders; how soon the varnish was applied; what conditions the painting is subjected to; and so on.
That being said, it stands to reason that a single layer of paint being less complicated structurally will have fewer interfaces where failure could occur. Painting conservators know very well that glazes are the most vulnerable to accidental removal and damage than solidly painted passages, especially if the glazes contain soft resins or balsams.
Looking at studies and sketches done by artists going back to the Renaissance, it appears that direct painting was probably employed by various artists in generating reference material to aid them in the execution of important paintings that were to be executed following more sophisticated techniques involving multiple layers.
To be developed in the near future.