Quality breeders have the hips of their breeding stock x-ray'd and certified by the O.F.A. Pet stores and backyard breeders don't do this. Hip dysplasia is very expensive to correct, if indeed the disease is not too advanced. Thyroid is easily treated but is often misdiagnosed as allergies, and it can cause aggression problems in some Chows. This causes irritation to the eye and could lead to blindness if not treated with surgery. They usually respond to a strong alpha person and hence the myth.
Chows are devoted to their family.
A properly bred Chow will stand in the face of danger to protect it's family. Chows do not bite any more than any other breed, especially compared to lap dogs, who bite all the time, but without consequence, so people think that doesn't count. But when Chows do bite they can be fearsome and this makes headlines. Chows are stubborn. Chows are not kissy lap type dogs.