I just found this HBR article, which recommends using a transparent strategy when giving negative feedback, and points out some more flaws in the sandwich method. I cannot comment on the scientific rigor underlying the article since I have not looked into that.

The author may have some interest in promoting his book through the article; or he may have an interest in promoting truth through his book; or both. Who can tell anyway?

I consider it curious and worth taking a deeper look at, at the very least. I hope this is useful to you, Pinecroft! smile

https://hbr.org/2013/04/the-sandwich-approach-undermin/