We dwere on the Viking Europe last summer, supposedly from Amsterdam to Vienna. We had to take the bus for the first few days because of low water and eventually got on the boat at Nuremburg. Lots of complaints, but Viking eventually treated us very well. We got a 20% refund on the fare and a certificate good for 50% off on a future cruise.
We are using the certificate in September on an Elbe river cruise from Berling to Prague.