They're probably referring to particular breeds that are known for chronic health issues.

Dogs with extremely flat faces (think pugs)? They have a lot of trouble breathing. Also, their eye sockets are shallow and so their eyes will pop out of the socket sometimes. There's a litany of health issues related to chasing the breed standard for dogs, sometimes related to how the standard has been taken to an extreme that deform the dog and sometimes related to the massive inbreeding that's happened as a result of chasing the breed standard.

There are certainly some dogs that can live in the wild, and there are certainly some breeders who try hard to be responsible about how they breed for the standard and how they breed their lines together, but the average purebred dog is not one you can just drop in the woods and it'll go find its own food and survive.

...not to mention species of wild dogs on more than one continent!

Nah, that at least is a fair point. Stray dogs mostly scavenge. They aren't actively hunting for the most part. My cat meanwhile used to catch at least 30% of her diet, something I know because she liked to disembowel small woodland creatures in the bathtub, where they couldn't escape.

I guess raccoons and vultures also can't live in the wild, then.

"Dogs" is also a really big category. I think a German Shepard might be infinitely more suited for living the wild, than say a Chihuahua.

My chihuahua was rescued on the streets of San Francisco after an unknown period of time fending for himself. He was a little underweight but not dramatically so. No signs of previous ownership (no chip, no collar, not neutered) so he was possibly just dumped somewhere at a young age (they estimated he was 1-1.5 yrs when found).

He fiercely guards food and will snap at anyone that reaches for him. He hunts insects including flies and cockroaches (he caught at least a few of those on our walks).

I don’t think he’d be as effective as a rat terrier, but I suspect he could hunt and kill field mice and maybe small rats.

Chihuahuas are one of the most common breeds found in Bay Area animal shelters (along with pit bulls). Lots of unethical backyard breeders around here who dump surplus puppies.

German shepherds are interstingly often very inbred. From talking to my girlfriend (this is her field), it sounds like it's unlikely that any purebred dogs would be able to adapt well to an actual wilderness

I have a pack of dogs. And they all hunt opportunistically. There're plenty of (mostly eaten) prey corpses strewn over the acreage. Anecdotal I know but I'm sure you can read up on prey drive in numerous dog breeds.