I had a Bluespark on my Jag X type and it properly pissed it off. Removed it, all was well. May have been a failed box as it did work for a while.
Your mileage may vary - literally. ECU remap may be nobbled by a Merc service if they touch the ECU, but otherwise should be good, if custom to the car. Generic maps are generic....
Brabus boxes are expensive, but talk to more sensors directly and are supplied by Merc, and can be warranted (to 60K miles) so what does that tell you? They also adjust to the heat/state of the car due to the sensor linkage.
In any case, mind your insurance