It's the hardness and weight of the hammer vs. what you're hitting and how hard you want to hit it.
From hardest to softest:
Brass
Copper
Lead
Plastic, wood
Rubber
Rawhide
I use my copper mjolnir most often for driving bearing races on motorcycles. The bearing races are hardened steel, and copper is dense enough to deliver a mighty thwack, but too soft to damage the steel. Copper is malleable enough that it holds together pretty well and can do its job with some marring, but not usually much chipping.
Brass would also work pretty well, but it's a harder than copper and it can be a little more brittle. Bearing races have sort of sharp edges, so a brass hammer might mar a little less, but could crack.
Lead would also work for this, but it's much softer and would damage the hammer a lot morem with chips flying everywhere. Plus, you know, lead, brain damage, etc.
A plastic or wood mallet wouldn't damage the bearing races, but would be too light to do much good. Rubber is a little heavier, but still not heavy enough and will bounce off; it doesn't deliver as much energy into the target.
As with anything else, you have to pick the right hammer material and weight for the job at hand.
As to the original topic... I have no idea why Germans might not prefer brass hammers.
