Not just metamorphic. It's typically a hard line between different rock types (exceptions for VS would by claystone, sandstone, and conglomerate, since those are grain-size/structure-defined irl and not mineral-defined), so the vanilla is going to be more technically realistic, since it treats all rocks as essentially different mineral types. But I very much understand the preference.

Here's a site that explains how sometimes you can get gradual gradients, but typically (for a transition between one rock type to another), you'd have sharper gradients. 

But if we are descriping the transition. The transition is smooth instead of abrupt. This is what is called smooth here, the transition.