The credit card thing is also true too. Cash is king here and I routinely see people with hundreds of Euros of cash like it is nothing. I always see people paying for groceries with 100 Euro notes. Furthermore, I know Germans are much more concerned with personal security and privacy that there is also going to be a push back about having to share personal information with an American company.
Also I live in Frankfurt, and, to be honest, I have no idea why people even use taxis. To get from the Hauptbahnhof to practically anywhere in the city is only a < 20 minute U-Bahn ride. Also, the public transportation in Europe tends to be insanely good and efficient so a ride-sharing app will also have to compete with this as well.