I don't see how you can make the leap from "its hard to measure" to "no metrics are useful". As with anything you have to use your judgement, experience, and its a case-by-case thing. Everything is a signal. Lines of code, number of bugs fixed, number of bugs found, severity of bugs, hours in office, meeting project milestones, contribution in team meetings, etc, etc. Its up to you how to interpret each signal. As you rightfully said, there is no silver bullet.