Harley, I own two dogs. A Kelpie X Border Collie and a Jack Russell X Maltese.
The Jack Russell X was bought for me when I was 10 from a pet shop. I have no doubt he was from a puppy mill now. He has bowed legs. And Jack Russell X Maltese? Worst breed combination of all time.
When I think about it, im so lucky he has got to age 11(12 next week) and had no genetic disorders come up. The Kelpie X is from a shelter, and at age 8, she also hasn't had any genetic disorders emerge.
But you know what, its incredibly lucky.
Thats all it is, luck. You may get everything you want in a breed, but chances are you wont. If your mixed breeds are everything you want, then you are lucky, pure and simple.
People that go to the effort of health and temperament testing, arn't the type of people to go and X breed...