A friend who had two breds from the same parents (different litter) said that you can predict the personality better in breds, while with unknown mixes you can get a manic dog and that they all have behavioral problems.
As you might have guessed by now, I am very much not a dog person. And I have no place to judge her statement. But I can imagine that there are a lot of dog owners who think like that.
Btw I'm in Germany, so is the friend. There is some Nazi joke in all of this that I am too lazy to make.