Feral dogs tend to organise packs, although small, very quickly. In all areas of the world feral packs are basically mongrels. There are no larger amounts of any one breed in feral communities than any other (per capita of breed in the region). This would suggest that, apart from the obvious non starters ( hairless, toys etc.) that is it down to individual character, condition and age at the time the dog is left to it's own devices rather than breed specific. It is then down to nature and that particular dog to see who is the strongest and most adaptable.
It is widely accepted that the original dogs that are feral don't, as a rule, last long. Bitches have a better chance than dogs, especially if they are in season. It's the offspring, the mixed breed dogs that are more likely to survive, they not only have their instincts to rely on but the things that they have been taught and the wider genetic experience.