The German constitutional court declared voting machines unconstitutional for the simple reason that they don't allow people with just an ordinary education, no specialist knowledge, to ascertain for themselves that the vote is kosher.
The CCC actually tried to get them outlawed based on technical grounds, including that it's impossible to have electronic voting that is both secure and private ("can't prove to the Mafia boss how you voted" type of private), the judges listened intendedly and asked many smart questions, just to then turn around and say "yeah we barely understood that and we're practically all professors and it's our job, can't expect J. Random Citizen to do that between shifts".