The concept of a S@#! sandwich is very similar to the feedback process at Toastmasters-Commend, Recommend, Commend (CRC). Having delivered presentation skills training for years I understand how fragile and sensitive people can be concerning feedback for public speaking which is one of society's greatest fears. Therefore the CRC feedback model and toastmasters is entirely appropriate.
The concern with using the CRC or the S@#! Sandwich model for courageous conversations, is that some people may find the mixed messages confusing. In fact, people who are very “below the line” in their attitude (defensive, slow to accept responsibility, quick to make excuses or shift the blame) may use the positive messages as an ‘escape route’ before accepting full ownership for the issue being raised.
Before deciding to use the CRC model in a courageous conversation first ask yourself “Am I doing this primarily to make myself feel more comfortable? Will this person potentially use the positive feedback as a distraction from accepting responsibility?”