Ages ago (back in the dawn of the modern breed in the 20s-30s in Germany), good quality brindles were hard to come by, and I know there were definitely structural differences (?weak toplines, I think?) that were more common to brindles than fawns, but that was probably due to being so close to the original one or two brindle dogs who may not have been particularly good specimens. I think Frau Stockman was one of the few who really worked to create good quality brindles, because many people wrote them off as inferior.
In the past 70 years where descendents of those original dogs were bred fawn to brindle a zillion times, any differences are long since gone. You COULD see a problem locally if a top local stud dog/kennel has atypically large dogs and produces a lot of fawns, but that would just be a fluke.