The unhappy corollary of this is that the metrics will not only tend towards simplicity and ease of measurement; they will also tend to reward mediocrity. Again, this is not an accident of history. A collective bargaining unit run by a “majority rules” system is always going to look for a system that rewards the median or modal worker, not the best.
Megan does an excellent job of describing the incentives driving unions. The structure of unions, not the individual members, pretty much determines how unions have to behave. It doesn’t take a rocket science to see why public education can’t improve much as long as teachers belong to unions.