I am going to wade in here even though I must admit I don't have engineering degrees or a wealth of technical knowledge by way of the automotive engine.
First, I do agree that the GM engine is a great piece of work-we have had several in different configs over the years. There is an unmistakable feel associated with the fact of getting in and going. The handling soon plays out after time and one feels things getting a little sloppy.
Second, having owned and driven a few MB's there is this sense which speaks to the driver and says: precision. In the generations following each other weight might have declined but handling performance remained static or improved in this relation. The fact that an engine is balancing itself vertically instead of in the V configuration lends IMO the subtle yet dynamic sense of balance which is counterpoint to the constant forces being exerted by changes in direction of the frame and body. As a result if the forces inherent in the inline engine remain overwhelming this gives the driver the feeling that there is a continous force maintaing the balance needed to remain upright.
I asked for and received the wonderful book ' 100 Greatest Cars ' for X-mas one year. Almost without exception the sense of vertical balance comes thru in a way to promote stability and this is made so appealing by the coachwork produced by the great car makers of the world over time.
Turner