This doesn't seem right. A good model necessarily leaves things out; if you didn't need to leave out some details, then you'd use the object itself, not a model of it.

Unless it was more expensive to build the object than to build the model. Or if the design process required information in the model that is not obvious in the object.

But the true answer is "I meant 'good' in the sense of accurate to reality, not in the sense of 'computationally tractable'."