Very. They put down the dogs with severe behavioral issues (e.g. biting). My local shelter is very good about providing adoption/surrender notes as well as behavioral notes. I decided against adopting a BC mix as they were clear exactly about the time/effort needed - he had been adopted out and resurrendered by a family who was exercising him 3 hours a day and they still couldn't calm his extreme (even for such herding breeds) leash reactivity. The volunteer explicitly recommended against adopting him.
Additionally, I've developed a relationship with a rescue by fostering for them over the years. They exclusively foster-to-adopt, so everyone gets several weeks (more, if they prefer) with their dog before signing papers. Most rescues at least allow you do do that, and I think people should take advantage of it.
I get what you're trying to say. I'll be the first to admit I'm a bleeding heart, and I feel awful for all the surrendered dogs in shelters - especially the ones who've endured abuse by their owners. But I place more stock in being able to live with a dog directly, instead of relying on their genetics. Sites like Petfinder mean you get a selection of dogs across the whole country. Additionally, casual pet owners aren't going to know the difference between a quality breeder and a backyard one. It's genuinely hard to tell unless you know what to look for.
My personal advice for shelters vs breeders is just "don't." People who do need to buy from breeders already know they do, and I'm not at all objecting to that. Everyone else is usually looking for a companion dog, and can find one at a local rescue/shelter.