When looking to purchase a puppy, reviews are good to read, but you still MUST do research on breeders.
NUMBER ONE most important thing to do in my way of thinking is to ask the breeder what genetic testing they do on their breeding pairs! If they don't do any testing or cannot produce results of testing, RUN don't walk away. I rescue yorkies and have seen more than my fair share of genetic issues that some people seem to think are not a big deal. They ARE a big deal....some cause severe illnesses, costing the owners a lot of money and causing pain and stress to the poor pups. Some kill them! Some of the medical issues don't yet have genetic markers but if it is happening in specific breeds, trust your instincts that is most likely is genetic! BUT there ARE genetic markers for many and you can read about them at the link below.
You can read about genetic testing on yorkies (and other breeds as well) here:
https://www.ofa.org/diseases/dna-tes.../all-dna-tests