Are all metaphysical claims flawed since they can't be proven? What's considered a flaw in a metaphysical claim if all of them can't be proven? Are there different types of flaw?
What's a metaphysical claim? Incapability of proof would not per se make a claim flawed - few claims can be proven true, and scientific claims are not among them; the latter are trustworthy to the extent that they account for the evidence. A shorthand (!) answer, then, might be that metaphysical claims are flawed because they are indifferent towards the evidence, that is, because you can neither disprove them empirically. In other words, they teach you nothing of interest.