Hybrid vigour kicks in at first cross. It's why so many highly productive farm animals are first crosses.
Also, if you cross two breeds that suffer from different recessive illnesses, the puppies are very, very unlikely to develop these illnesses (you'd have to very unlucky and have not only a carrier parent from the breed that often has it, but also a carrier parent from the breed that has no/very few recorded cases). It's obviously different with things like hip dysplasia, which have a very unclear pattern of inheritance, the observation of which is almost certainly confounded by environmental factors.
As an aside, I have a dog who came from an ad on P4H. We did our due diligence (as did the breeder, who was very thorough, and who kept in touch for about five years), and the dog in question is a really excellent little dog (healthy, sound conformation, confident and environmentally stable, biddable, great worker: if I could have a clone of her as my next dog, I probably would).
I can see why P4H has a dodgy reputation, as it accepts ads from all and sundry, but if you take responsibility and check out the breeder, you can find some very good dogs on there. In all honesty I don't think ChampDogs is morally much better, given the eye-watering COIs of some of the breedings on there - we crossed several CD litters off for just that reason when we ended up with our P4H puppy.