It breaks down when you don't have scale (lots of things to measure) & you don't have a repeated process or output (measuring the same thing). Even worse, you're not measuring good quality, you're measuring poor quality.
If the focus is on making things less badly than you do now, then there is a limit to what can be improved, & most of the steps are incremental.
If you want to measure good quality, & improve it, then you have to take an entirely different approach - you have to measure something that is good, & achieve a higher measure over time.
When producing goods & services, then customer satisfaction is often the best measure. This is, of course, also the most dangerous, because it's subjective & dependent on who your customers are at that point in time, the last job you did for them, etc.
But, & this is important, if your customers are regular & keep coming back, & every job is to the same exacting standards of high quality, then the customer will consistently see your goods & services as being of high quality, & will continue to be good customers.
Somewhere down the line, if your customer is not an end user, they also do customer surveys that show them how their products & services are working in the marketplace.
The next problem with measuring customer satisfaction is that you generally can't do it yourself - it requires some arm's-length work by another organisation or individual, which costs a lot more to implement than sitting on your production line & counting the reworks or defects.
Nobody said that quality was going to be an easy thing to implement.
It definitely doesn't have to be easy to measure.
The question really is, how meaningful is anything but the subjective response of your customers? You can have the highest auditability of product, with zero defects, & still be commercially in the red because your potential customers don't trust you or your ability.
You won't know that unless you survey them.
No measures of improved processes & lower defects will change their opinion - or affect their satisfaction level.
