The danger of developing a qualitative metric for work/job performance, is you now have a recipe to game the system for anyone that is so inclined. Of course, if this metric is used as an indicator for promotion and raises, nearly everyone will be inclined to game the system.

It might not even be as overt as that. It is a well known phenomenon that what you measure, you improve.

So as soon as you have a quantitative metric for work performance, almost by definition, it can become problematic to use, unless you are very careful how it is used.

This is not to say that finding a qualitative method is useless, far from it. It can lead to valuable insights and improve overall quality of the team/business.