I hold a very different position to the one you suggest. I am a professional engineer having worked in the medical device industry for 20 years now. I have had plenty of experience with (among other things) proving designs are fit for their intended purpose - a necessity required by regulatory bodies worldwide.
Attempting to test a product or a process by focusing on the individual sub systems would never cut the mustard. Along with low level testing (known as Verification testing) to prove that sub systems meet their functional/engineering requirements, it is imperative to also perform testing at the system level - this is known as Validation testing. Validation testing is performed without needing to know any internal implementation details. It is akin to black box testing. It proves the product meets the needs of a variety of stakeholders such as those of the customer and regulatory bodies (such as the FDA).
As a photographer if all I did was individually test the sub sections in my photographic process I would only ever be performing verification testing and not validation testing. Both are necessary and valuable.