The worst of it is that the stated rationale for doing them ("Without a formal measurement, it's hard to know if you're reaching your goals and meeting the expectations of your board.") is such a crock. In organizations where this is true -- and they are legion -- the appraisal process is even more stress-inducing and destructive.
I like BoardSource usually, but selling a pricy canned assessment package and then saying "BoardSource recommends appraisals every 12-18 months!" is so perfect -- kind of like the illustrations of conflicts-of-interest that appear in BoardSource books.
If you're aghast that someone would challenge the entire practice of performance appraisals and can't imagine what you should be doing instead, you should read this enlightening work.