It only seems as if the more 'natural' breeds are healthier. If you DO NOT test, which is the case in the breeds with 'no recorded health issues', the you cannot know what is out there.
This is why people insist that mutts are healthier with less genetic diseases. This is not true, the only reason there is no record of health issues in mutts is because you cannot OFA a mutt you cant CERF a mutt. And what RESPONSIBLE mixed breed breeder will health test their dogs before breeding? I bet you can't find even one.