In the last five years, I have taken taxis in a number of German towns:
Lindau (multiple times)
Berchtesgaden (multiple times)
Fischen im Allgäu
I don't think I've never been overcharged. (Of course, since the highest of the fares was probably 10€, it would hardly be worth overcharging.) Admittedly, except for Munich, these were all small towns, and most were to/from train stations, not airports. Maybe scammers only pick airports and large towns. Or maybe Germans are just more honest.
In Munich, the driver took a somewhat circuitous route of over 1 km for what would have been a ½ km walk, but I checked it out on Google Maps when I got home, and it was the shortest route by car due to one-way streets and pedestrian zones.