What if, however, I'm an engineer that's working on a new piece of machinery? This is a project that requires the involvement of many highly skilled workers, electrical engineers, mechanical engineers, computer programmers, etc. I might be responsible for a single system or even component, but my work directly relies on several other's work, and it may change depending on the circumstances (say the mechanical engineer has designed an arm that would damage a critical electrical system). Now you've got a moving target. One day your work was x, now the circumstances shifted and it's y. With potentially hundreds of people involved it would be very difficult to asses what any one person's contribution was based on your standard above. Indeed, if you could asses that from the beginning, you wouldn't need all those engineers to design your machine since you had already done that work.

This generally means that you're better off being less efficient while still meeting the deadlines/goals.

Of course, there's a tradeoff since proving yourself to be more efficient than expected could result in more work etc, but it's not as clear cut as being rewarded for efficiency.