Do you agree with breeders that ask you to keep a certain breed ONLY on a certain brand of food or ONLY on a raw diet etc?

If it's a raw diet I agree, then. But it's true for all dogs and even cats, not several particular breeds.

If it's a particular brand, then I'd go suspicious and would rely on my knowledge, Google and common sense.

I haven't met such breeders but if I had I'd ask him many questions about why he thinks like that!
It sounds a little nonsense to me. Every dog is different within the same breed, some are more active and some are less, some have better health and some worse. Feeding them all the very same thing doesn't meet an individual dog's needs. Although sometimes these needs can be similar, they are never the same.