I live in a severely mountaineous region with incredibly twisting roads and out-this-world gradients.
Both, the owner's manuals on my cars AND the local service manager at the dealership strongly recommed to downshift manually your automatic transmission to conserve the brake pads.
If you do not do this, brakes pads will wear at a rate of 8,000 miles (or even less!!!) because of the constant braking.
My S 500 L will automatically downshifts for me most of the times, which is nice and convenient. But there are some instances that not even the electronics of the W140 will serve me right and I have to manually downshift to get proper engine braking.
The Mercedes-Benz auto transmission was especifically designed for this purporse and it wil not, in any way, affect reliabilty or durability. Now, drag racing is another issue. Any transmission, manual or automatic, will suffer badly from over abuse, it is just plain logic.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995