We did our first Viking River cruise from Amsterdam to Budapest on Viking Sky in 2011 and could not fault anything. The food, cabin, cleanliness, staff, excursions and the boat was all very good, so I shall be basing this report on Viking Rurik against the Viking Sky cruise.
To start with the over 2 hour coach trip from Moscow airport to Viking Rurik's berth was very tiresome especially after a 18 hours of travelling from West Australia. The EX cabin which was the largest on the Main deck, but we latter found out when we cast off was also one of the noisiest because of what I suspect are the side thrusters used to berth the boat & position it in Locks. This usually happens in the middle of the night (I even rang Viking, before we booked this cabin to find out any down side to this cabin and none was mentioned).
Other areas which I thought was not too good:-
Functional, tidy, always clean, good layout, but a bit noisy