Its nothing to do withem being f1 or 2nd 3rd or 4th crosses.Its down to them being bred from shit quality animals in the first place.If defects are showing in the second cross they were there in the f1s and in at least 1 or both the origional purebred parents.If I was going to buy a cockerpoo of labradoodle which I aint Id want to see proof that the purebred stock ie poodle,Lab,cocker had all the relevant health tests associated with that breed