A competent cam regrinder should be able to convert mm to inches without much of a problem. All that I have heard of, when it comes to cams breaking, is when you either go to far with the regrind, or the cam itself is a poor batch with casting and material flaws. there are three cam companies that I would trust with a regrind or new cam. isky, crower and comp. those three have been around a LONG time.
(I've seen and heard alot about newer comp cam, cams going flat recently though)