That’s just a load of horse-hockey that older mechanics say because they hate modern vehicles. Techs are not electrical engineers just because they understand how a multimeter works and many places will just replace harness and stuff wholesale instead of repairing them, to the point where if I want to make one of my side signals work again I’m going to have to cannabilize a parts car for that section. Even still, it’s just lining up the wires and busting out the solder and heat-shrink.
I have a 2015 BRZ and do the work in my parents’ driveway. I know my car is less complicated than an Audi or VW but from personal experience those cars are garbage designed by engineers of questionable skill but a large budget.
The main reason independent techs have a hard time, I’d imagine, has more to do with how weirdly packaged the wires could be and without manufacturer support(for a warranty job, say) it might get financially difficult. Unplugging harnesses is not that hard otherwise.