This study published data from 50,000 DNA tests (F adjusted column - Additional file 1: Dataset 1)
It does note that:
There were interesting exceptions to the correlation of inbreeding and health. The Border terrier, Basenji, Collie, and English setter breeds have high inbreeding but low morbidity. Likewise, the Malinois, Pomeranian and Russian Tsvetnaya Bolonka (Russian Toy) have lower inbreeding and high morbidity. These example breeds are neither brachycephalic nor particularly known for extreme morphologies. In the case of healthy breeds with high inbreeding, it may be possible that these breeds have been purged of deleterious alleles as has happened with inbred mouse strains [46]. In the opposite situation (lower inbreeding and high morbidity), the recorded morbidities could be high allele frequency Mendelian diseases or potentially conditions linked to phenotypes under selection in the breed.
You can also look at the plots in the study and see that while a high degree of inbreeding is associated with considerably higher mortality, there's a huge amount of overlap. You can do dog eugenics and get very unhealthy dogs, but you can't do dog eugenics and get dogs that will never have any health conditions (except the worst physiological ones that only exist because of ridiculous amounts of inbreeding).
They can be, it depends on their ancestry
This study published data from 50,000 DNA tests (F adjusted column - Additional file 1: Dataset 1)
It does note that:
There were interesting exceptions to the correlation of inbreeding and health. The Border terrier, Basenji, Collie, and English setter breeds have high inbreeding but low morbidity. Likewise, the Malinois, Pomeranian and Russian Tsvetnaya Bolonka (Russian Toy) have lower inbreeding and high morbidity. These example breeds are neither brachycephalic nor particularly known for extreme morphologies. In the case of healthy breeds with high inbreeding, it may be possible that these breeds have been purged of deleterious alleles as has happened with inbred mouse strains [46]. In the opposite situation (lower inbreeding and high morbidity), the recorded morbidities could be high allele frequency Mendelian diseases or potentially conditions linked to phenotypes under selection in the breed.
You can also look at the plots in the study and see that while a high degree of inbreeding is associated with considerably higher mortality, there's a huge amount of overlap. You can do dog eugenics and get very unhealthy dogs, but you can't do dog eugenics and get dogs that will never have any health conditions (except the worst physiological ones that only exist because of ridiculous amounts of inbreeding).