Car & Driver really wasn't doing its job on this one. The better comparison to the 2015 ML250 BlueTEC's 2.1L twin turbo would have been the I-4 biturbo to some of MB's venerable I-5s and I-6s of yesteryear. The worlds 1st Turbodiesel (300SD), was an I-5 and reached its highest state of tune in '85 at 125 HP and 250 Nm of Torque.
Granted, MB's 3L V6 turbo diesel is in a league of its own, no question. Even so, MB's I4 twin turbo diesel (OM651) is impressive; and, it's true, C&D does see that --e.g., "Highs... torquey diesel engine." Still, it's actually pretty amazing compared to previous diesels that powered some of MB's premier flagship sedans that came in at about the same weight as MB's modern-day SUVs. For example:
MB's '91 - '98 SE/SEL/SEC models (coming in at nearly 5K pounds) were powered by a 3L I-6 diesel (OM603/OM606) with turbocharger and intercooler that put out, as follows: 150 HP @4100 RPM and 310 ft-lbs Torque 2000 RPM for the '603' / 174 HP 4400 rpm and 243 lb-ft Torque @1600 rpm fpr the '606,' or, a torque falling from 420 Nm to 330 Nm to achieve the greater overall horsepower.
By comparison, the new 2.1L bi-turbo diesel comes in, as follows: 200 HP @3800 RPM and 369 ft-lbs Torque @1600 RPM. Looking at just the HP the little diesel clearly blows them both away. Interestingly though, when you look at the torque, MB's I-4 easily handles even the torquier but older '603' (310 ft-lb/420 Nm): the I-4 puts out 420 of its max-500 Nm from about 1300 to 3K RPM. The solid blue area of attached torque shows how much additional twisting power the little diesel has over the old '603' engine.
Perhaps the 2.1L will be more boring than MB's modern-day 3L turbo diesel (OM642: 240 HP @ 3,600 RPM and 455 ft-lb of torque @ 1600), that compares with a V-8 gasser in performance; however, as the Irish reviewer Bob Flavin said when reviewing the 2013 ML250 that was available in the EU at that time, it is brilliant! That was a brilliant review!