Gem is totally clueless. This BS of "hybrid vigor" has been disproven many times. It is the line of crappola that lousy breeders use to justify breeding MORE dogs for which there are not enough homes.
Myth of Canine Hybrid Vigor
Just so you do not assume that I do not know what I am talking about, I am the current president of a Labrador rescue and have worked actively in rescue for more than 20 years.