Certainly there is a difference between sixt in germany and the other countries (not even going to start whats happening at sixt in rome), but in germany all the things mentioned above are very unlikely.

In contrast to europcar and hertz. I had 4 rentals at hertz this year, every single time I had to complain so that in the end the final amount was reduced. They even told me once that they dont have a c class (which I booked) and actually wanted to call me that they cannot give me a car. appearantly they found a focus cmax for 1e less. they actually expected me to be happy.
I certainly had problems with sixt, but not this kind...