Upvote:2
At least at first, the German Army in WWI was organized by districts in the country - so a unit would be full of Hessians, Westphalians, or Prussians. This extended to the extent that nearly all of one of the armies attacking France (the 6th) was made up of Bavarian troops and was led by the Crown Prince of Bavaria. Later in the war they might have h*m*genized units by drafting replacements from all over rather than keeping up the purity of the unit.
This would limit the ability of a unit commander to use less favored ethnic groups for risky missions, since there wouldn't be any such troops to speak of from a district far removed from such persons.
I don't know that Germany did the kind of unit shuffling that more diverse nations like Russia and Austria Hungary did - AH sent Slavic units to fight Italy which they did with gusto rather than versus Serbs or Russians where they were less willing. I haven't heard that they did, but their ethnic issues were far less severe than the other two nations mentioned.
A unit from a district with dubious reliability might well get lesser jobs than a more reliable one, but this would likely be digging ditches in the rear rather than leading risky attacks. A bad unit on a risky mission is more trouble than it is worth - you might as well shoot the fellows than give them a chance to rout and lead to a major defeat in the field.
So to sum up, I have heard of these exact problems in the context of most other nations in WWI rather than Imperial Germany, so I doubt the issue was severe in their case.