You'd be surprised how often I have unearthed bugs inside chips. At first it's "Oh, no, that can't be, that must be your layout". A few iterations and some bench tests at the manufacturer later ... "Oh s..t!"
Board level design isn't that tough. Ok, costs time but the layout is not expensive. But then you know what it does and where its limits really are.
Even worse are chips with the power devices on there. That's like a seismic sensor on a Harley engine.