There is some subjectivity built into every stat.
I have often used strikeouts as an example. It's probably one of the most, if not the most, subjective stat there is. Just consider all of the complaints there have been in the game threads regarding the strike zone. Yet people don't seem to have any problem with trusting the reliability of strike outs, or walks for that matter.
However, when it comes to UZR, the thought by many is that it can't be trusted because it's subjective.
Why is that?