The trouble with the sandwich approach is that anyone who has been involved in courses where it is applied (as in any and all Arts courses) can spot it at 100 paces. It is possible to do it reasonably well, but that skill is only acquired with years of experience, and even then, if something is really bad it's hard to hide the criticism filling between slices of lukewarm praise.
Someone I know who goes to poetry workshops, for instance, often goes on about the sandwich technique, and really believes she applies it, but might say 'I loved the topic of the poem, Gladys, but wondered about the way you expressed it, and thought the structure needed work. Overall, though, the poem was interesting.' Well, she probably wouldn't say anything quite as bad as that (I am making a point), but I hope you see what I mean?
IMO there are other factors - one is that feedback needs to come from someone who knows what they are talking about eg what sells, or what appeals to different markets, what is in vogue nowadays etc (in other words a publisher) and another is that the author is experienced enough at getting feedback to get benefit from it, and many are not.