The reason anything is so difficult to measure under the capitalist mode of production is simple... corporate subterfuge.

Here's a corporate talking head caught in a moment of (highly) abberrant honesty -

"We are not technically in the food business. We are in the real estate business. The only reason we sell fifteen-cent hamburgers is because they are the greatest producer of revenue, from which our tenants can pay us our rent.” - former McDonald’s CFO, Harry J. Sonneborn

What this means is that the employees (in the case of McDonalds) can devise a million and one methods to measure say, the quality of the food or the experience customers have... none of it will really matter because the capitalists at the top doesn't actually care - the actual measurements they care about is rarely communicated down the line with such honesty for very obvious reasons... they want their employees and customers to believe that McDonalds actually gives a damn about the stuff they sell people. The truth of the matter is, they don't.

Measuring and evaluating things isn't rocket science - it's something people have been doing for centuries. But measurement and evaluation is only possible if the people in power wants something to be honestly measured and evaluated.