The M-Class has independent suspension all around. Very sophisticated. It makes for superb on-road handling and crisp performance. Off-road however, wheel travel is essential for traction management. For better understanding, please read the section about springs and traction on my 4WD101 site.
Even though 4ETS is a brilliant traction management system, it is more an on-road safety feature than a capable off-road tool. That's why the G-Class employs manual differential lockers for traction management.
Bottom line is, when the going gets rough the M-Class will get stuck where other SUVs without traction support (Jeep Grand Cherokee for example) don't even show signs of wheel spin.
An advanced 4ETS program from model year 2001 on handles traction loss due to lack of wheel travel much better. However, this improvement works only in low range.