The worst part of being a manager in my old job was the employee review process. Every six months I had to evaluate the members of my team and assign them a rating. The rating itself was a combination of a bunch of subjective ratings I would assign, plus some metrics that were supposed to show objective things like numbers of calls taken or whatever.
Apart from taking a huge amount of time to gather all the data, collate it write it up a cogent review document and deliver it to the employee, the employee was often confused because the system would change at least once a year. Of course you wernt allowed to have a team of great people, it was a relative system, so even if Joe did a bang up job all year, if Mary did a little bit better, then poor Joe gets a lower rating.
Even if people did understand, I literally never saw it change anyone's behaviours; the bright compliant ones, carried on being great to work with; the ones that struggled didn't do any better for being told they were struggling.