Yes, first generation 'hybrids' can be healthier than the purbreds used to make them, but second generation is where you have problems.
If you have a Poodle with the PRA gene and breed it to a Labrador that doesn't have it then the puppies will NOT have the gene... but they can be carriers for it.
So if you then breed that puppy to another carrier there is a good chance the puppies from that mating will end up having PRA.
So the first generation of "Labradoodle" is clean, no problems. But the second generation has big problems.
Good breeders also do genetic testing on their dogs to try and eliminate any problems the dog may pass on. So problems like PRA with reputable breeders are non-existant.