bethoo - load of rubbish, i'm afraid..
a good breeder will attempt to breed out these genetic diseases (hip score, eye cert, Seb a. skin punch, etc etc appropriate to breed)
classic example = progressive retinal atrophe which has been all but eliminated from miniature poodles.
too long x-breeders have claimed that the dogs they breed can't get these disases - out and out twaddle. especially as if both (for example) labs and poodles can get hip dyplasia, how does crossing them help? if, on the other hand, you hip score two dogs and check their hips, then breed them only if they are ok, chances are the offspring will have good hips.
shows an extreme want of understanding of the science of genetics.
too often a myth used to get people to pay top dollar for dogs without pedigrees, paperwork / KC registration or any other minimal safeguard to the welfare of the dog.
i would be very suspicious of any breeder that claimed their puppys were clear because they were crossbreeds,rather than they had done all the necessary testing and research into the pedigree.