All of the river cruise offerings in Europe are good- very good in fact. They occupy a niche akin to the likes of Azamara or Oceania, if one wanted to reference ocean-going ship experiences.
The components are straight forward, as the dictates of rivers and locks pretty much constrain the hardware.
Ships are all virtually the same size. So the Viking Longships are the same as those of Uniworld or AMA or Avalon or Scenic or Tauck or A-Rosa. The layouts do differ, and stateroom sizes/attributes as well.
Small cabin, bed could not be made up as twins. Would not recommend the first level cabins on any of the River Cruiseships becuse of the small portholes and no sliding door/french balcony. Good storage and functional bathroom.