I haven't done Amsterdam to Basel, but I returned 2 weeks ago from a cruise down the Rhine from Cologne to Speyer, south of Mannheim.
We went with Leger and one of the pluses for us was that they would collect you from your door to where you joined a feeder coach to Folkestone where there was a grand interchange onto specific tour buses. The plus here was that once the taxi arrived at your door you did not have to lift or carry any siotcases until you got home again.
It was a long journey, 5 hours from Dunquerque to Cologne. The cabins were excellent, we had top deck ones with the sliding door/windows. The service was excellent.
It was not very dressy, some people did go a bit cocktaily in the evening, but for most of the holiday I wore sweaters and jeans/cords and I dressed for dinner by changing out of trainers into a nice air of shoes and adding a necklace.
The big plus for us was the lack of handling luggage. DH is quite infirm these days and some days cannot walk very far. Knowing that baggage would always be handled by others was a big plus.
The advantage of a Rhine cruies over a sea cruise is that there is so much to see as the boat moves along, I could write an essay about the importaance of the RHine in European trade after a one week cruise, but will not bore you.
DH went down with a respiratory infection after 4 days (not, I think anything to do with the cruise), but I could leave him happily on board sitting in the lounge with a book, other people around him and wandering back to the cabin when he felt tired, while I could go out on outings or to walk round towns we stopped at.
The food was plentiful, but portion controlled and I came home the same weight as I went away. There was a buffet breakfast and each evening you made your choices for the next day's menu from a menu sheet provided.
I found the heating on board (22-24C) too hot for my tastes, but in our cabin we turned it off and had our huge window open all night.
For me the downsides were mainly that the river side places where we stopped were not terribly interesting and the shopping was not good. There is a dearth of good guide books so it was difficult to engage with places because you had n context for them.
