90% of all dog genetic health problems are recessive. This means that in order for a puppy to have the health problem, both parents must carry it. By breeding two dogs of different breeds, without similar health problems, we are breeding puppies with far less health problems! Though, we searched for healthy parent dogs to begin with, if a dog shows signs of any health problems we no longer breed that dog.
Crossbred dogs have far less health problems, (hybrid vigor). True that the outcome is not as predictable as a purebred dog, though, we can predict a lot, by knowing the parents, past litters, and only breeding healthy dogs with great temperaments to begin with.