Open communication is always the best policy and, for sure, it's in the best interest of the organization to have satisfied volunteers. Perhaps some adjustments could be made to remedy the situation. Or, maybe it's simply a matter of the 'right' person in the 'wrong' job. Don't get discouraged if an assignment doesn't work out. There are lots of other volunteer roles out there, and you're bound to find something better suited to your interests and needs.
Most people become anxious about performance reviews, even if their work is outstanding.
But bear in mind that a formal appraisal is a sign that the organization is interested in managing its people as effectively as possible, whether they are salaried staff or volunteer workers. As a volunteer, you should get feedback on a regular basis and whenever you request it. A formal appraisal is usually done at the end of specified time period or when a project has been completed. Normally, this takes the form of a written report, which is then discussed with the volunteer. The thing is, an appraisal is meant to give you informative and constructive feedback on the service you are providing.