I was under the impression the 5.5l was a better motor? What's your impression of the M279?
The M157 engine 5.5L was the prev generation amg engine- the motor is extremely sensitive to fuel quality, rods are a weak point, timing components as well. Main bearings would go to shit also b/c of the ECO (start/stop) feature.
The new AMG engine the M177/178 4.0L is far more superior and robust. Have not seen many issues with those and they have been out since 2015. They are also a Hot V setup. Not sensitive to fuel quality.
Regarding the M279 thats a good engine. They have commonly failed parts such as intercooler pump, they eat spark plugs (2x per cyl), coil packs go bad, spark plug ignitors go bad as well.