Well I have quite a bit of experience with rescued dogs and I am also a responsible breeder and have the code that says if you are gonna breed, you must also rescue...
There's a bit of a difference between a breeder who rehomes dogs he or she had previous sold, and a regular rescue who just accepts any dog of a particular breed & scours animal shelters for any that happen to be put-up for adoption. In the latter, you really don't know any more about the dog, its health prognosis, or its past than what you can see right in front of you.
I have never had a bad rescue, but I actually know of more puppies that were purchased from a breeder who turned out to have so many issues it was ridiculous... So the cost for a good dog and the fact that you bought from a 'reputable breeder' does NOT ensure you will not have heartache of a puppy..... so to rubber stamp all rescues as being problems is simply untrue and ridiculous!
I wouldn't stamp all rescues as being problems, and I don't think anyone else would, either. It's just that you don't know what you are getting into, you're signing-on to be a forever home without knowing the first thing about dog's past in most cases.
The vibe I got from the couple of breed rescues I contacted many, many years ago (one of which was a GSD rescue, but I can't say I actually visited with anyone involved with rescuing Dobes) just didn't make me want to deal with them, either. I got the feeling that they intended to place all the blame solidly on my head and make me feel bad if I was to return a dog to them because the situation just wasn't working out. I tend to be overly sensitive on issues like that, and absolutely don't want to be in that position. It's not fair to oneself or the dog to keep it out of a sense of guilt, obligation, or shame if you're just not bonding, but that's sort of the corner some rescues seem to put folks in.
You're absolutely right, of course, that you have no guarantees with a puppy from a reputable breeder, but at least you know its past and that it wasn't the result of a haphazard pairing with uncertain genetic history, produced just to make a quick buck. And, well, people who buy linebred dogs from breeders who focus on those sorts of programs can see in the pedigree that inbreeding has occurred, and that health risks will be amplified-- caveat emptor.
Now, if you had rescues that weren't as obsessed with control and weren't so condescending, it would be much more appealing to help them out. I'll just throw this out there, rescues are asking prospective owners to take on a lot of unknown, potentially expensive, and potentially heartbreaking risks, while giving them more grief and demanding more control (such as those "your dog is never actually yours, it's our dog FOREVER and we're just letting you keep it for now, and you give us permission to inspect your home at any time in the future" clauses) than most breeders would give a prospective buyer, and that just seems dumb.
That said, my previous (and first) Dobe was one who had been returned to her breeder. The breeder knew a lot about her circumstances in her previous home, so I had a good idea what I was getting into. That was a win-win situation and it worked out really well.