I think the answer is here, from Lender Letter 2015-2.
"The risk analysis performed by CU is for exclusive use by the lender in their analysis of the appraisal report. After completing a thorough review, a lender should be able to have constructive dialogue with the appraiser to resolve specific appraisal questions or concerns. Although the lender may use output from Collateral Underwriter to inform its dialogue with appraisal management companies and appraisers regarding appraisals they supplied, the CU license terms prohibit providing these entities with copies or displays of Fannie Mae reports that contain CU findings, including without limitation the CU Print Report, the UCDP Submission Summary Report, or any other CU report. The lender must not make demands or provide instructions to the appraiser based solely on automated feedback. Also the CU license terms prohibit using it “in a manner that interferes with the independent judgment of an appraiser.” Fannie Mae expects the lender to use human due diligence in combination with the CU feedback, and will actively follow up with lenders who are reported to be asking appraisers to change their reports based on CU feedback without any further due diligence.".
They do not need a comment, until the UW has done his/her due diligence in regards to the quality of the appraisal report. If, after the UW has done their due diligence, they can ask for commentary. They CAN NOT just ask for comments in the appraisal report, as a matter of business. Due diligence must come first.
If an AMC is requesting, after the AMC ran the appraisal through the CU, you should contact FNMA, as AMCs are not allowed to have access to the CU.