As a general rule, building things as an engineering project (according to engineering, whatever you want to call it) doesn't prove they work. Rather, it proves they don't fail in specific known/defined ways. There's always ways they can fail that aren't known/checked, and frequently ways that aren't considered important.

Kind of like automated tests in software development. Except the people that pay for the engineering stuff are more likely to pay for the extra tests, because of liability.