Everyone needs feedback on what they do to get better at it. Good evaluation takes a lot of time. Resistance to evaluation is often emotional, which is not to trivialize it. If people aren't feeling good, nothing gets accomplished. The solutions, then, need to recognize this reality.
Leadership is crucial.
At least it gets rid of uncertainty which feeds anxiety. If the head is not fully behind the evaluation process, the process is doomed from the outset. Accountability is a professional obligation. Research shows that people are willing to be accountable when the process is thorough and fair. Pick your volunteers carefully and involve key others as peer evaluators.