The fact that a vehicle has four wheel drive does not say much about its off-road capabilities. The Mercedes-Benz M-Class is a light duty SUV with very strong on-raod priorities. The ML is a wonderful vehicle for travel and exploration in light duty off-road environments. It will feel right at home on most of the world's dirt roads. As long as the terrain is not too rutted and traction is good, like in the picture above, it even climbs 100% grades (45 degrees).
The Ml's medium off-road capabilities are quite limited and heavy duty terrain stays inaccessible for the ML.
The links on the left will give you an overview where exactly the ML reaches its limit.
Feel free to send me examples of your experience - I'll post them here.