Your response illustrates one of the problems with the Homebuyer Protection Plan. That being, many buyers, sellers and agents believe the appraisal is an 'inspection'. Your example of plumbing material is something most appraisers would not pick up during their data gathering visit to the subject property. Appraisers are to report on 'readily observable conditions' which may require additional inspection or repair.
Please be aware, the appraiser does not certify the condition of the property. The appraiser completes and appraisal to arrive at an opinion of value for the lender to make a loan and for HUD-FHA to insure the loan. There is no such thing as a 'HUD Inspection'. Any such reference is an error.