If a postulate is not verifiable, then you have a BIG problem if you treat that postulate as true. Because what you have is a mere assertion instead of a genuine postulate. Mere assertions live forever in that limbo known as “truth value unknown”, until [1] someone devises a proper test of that assertion, and [2] that test is conducted. As a corollary, treating mere assertions as fact, in the absence of proper testing thereof, means you are walking through epistemological fog.
Oh, and as for metaphysics, be very careful about deploying this term. All too frequently, we see here assorted specimens who think that tossing the word “metaphysics” into their posts, magically hands free passes to unsupported blind assertions. It doesn’t.
Another problem with the use of the word “metaphysics”, is that much of the classical variant of that part of philosophy, was devised before modern science, and in particular, modern physics, rendered some of the questions therein redundant. Indeed, modern physics has been, to use the words of one commentator, terrifyingly successful at validating its postulates about the observable universe. If you think you can sweep that success away with nebulous usage of the word “metaphysics”, you are in for a nasty shock.