> If so, your metric cannot distinguish between a bad engineer and a good one.

A metric that moves in the same direction and amount for everyone based on external event isn’t a problem. The delta in performance of the great engineer will outweigh that of the poor, since the metric movement that is due to external circumstances will be the same in each kind of engineer and thus not count.

That’s a new one. I have never heard of a company which operates like this, giving everyone equal reward no matter how much they contribute individually.