There are two golden doodles in my neighborhood and neither of them are very intelligent. It could of course be the breeding.
As for purebred's, my golden is 14 1/2 years old and still in good health. We keep her trim at 62 pounds and never feed her human food.
As for Cancer, all of the dogs that I know of that contracted it all ate human food on a regular basis. Not a scientific study, just an observation on my part.
Good luck with your research.