I see this posted frequently as an encouragement to steer people towards rescue.
In one way, it's right and I agree with it.
On the other hand? Well, the reason people chose a purebred pup is that it's not a complete crapshoot. They know the genetics, they have an idea of the genetic potential, they can see the health of the parents, they can see their tendencies and temperaments. If the resulting litter truly will be a complete crapshoot then one shouldn't be breeding it.
Is a well bred pup a sure thing? heck no! But, when buying a pup, we should be buying it because we have an understanding of their genetic potential and that potential is a good fit for our wants/needs. It's why I got Tess after living with Kolt for three years. It's why people see a dog they like out trialing and end up with a pup out of that dog.
And a rescue, while we know their outward, exhibited potential, we aren't going to understand their genetic potential in the same way that we might a pup. We may work with that rescue for years and still have a fearful mess. Or they may turn out fabulous. Or they may have a fabulous temperament and underlying health issues. Realistically they are potentially a crapshoot of a different type.
If you can solve the issues with a rescue, you can likely raise a well bred pup right. If you want something specific, find a well bred pup with the right genetics or a rescue that fits the bill. One choice isn't better or worse than the other. Both are about finding a dog that suits your life and needs.