Measuring outcomes is hard, especially in comparison to outputs which are easier to quantify but are different than outcomes. True measures of outcomes usually can only be tracked over a long period of time, whereas outputs will give instant feedback.
Look at what has been happening in the schools with standardize testing. Proponents think they are measuring outcomes (effectiveness in preparing children to be skilled & productive members of society) with standardized tests. They are actually measuring outputs (test scores) as a proxy for the outcomes.
The result is the system being measured will adapt. Schools "teach to the test" which improves the outputs but we don't know the effect on the desired outcomes. It may result in children being less prepared.
This is a well known effect in companies that measure individual performance in terms of outputs (widgets manufactured, # of upsells, etc). The employees will deliver the required metrics but it often comes at a cost to quality and customer satisfaction which is a negative outcome to the company.
Which is the main lesson in quantifying outcomes: Measuring the wrong thing can be worse than not measuring at all.