A metal actually needs to be of a certain hardness and elasticity in order to be polished. For instance, only Grade 5 Titanium can be polished, and that's due to the hardness. However, gold can be polished (it can be done in 24K but more normally done in 18K/14K) despite it being a soft metal due to its other properties.
So I think it's not just the difference in hardness, but it's generally said among watchmakers that brass is generally much more difficult to polish. Then the rhodium plating covers up a lot of the detail of the hand work on the brass. So it's not entirely a fair comparison to compare a German Silver movement plate vs. a brass one since the German Silver one can always be polished to a higher degree.