BulletProof parts are intended for those who value having all machined parts in their gun. They are designed such that they must be hand fit to the gun. In honesty, some parts are just fine made from cast or MIM steel -- the grip safety for example. But folks buying WC pistols want machined parts and hand fitting. They get that with Wilson Combat, even on the ACP models.
That's not to say that BulletProof parts are any better than the parts used by other high end shops.