I cannot speak for their North American cruises, but your impression is pretty much what I experienced and as far as differences between them and the cruiselines you mentioned I have been on Carnival, RCCL, and NCL and the difference is night and day. They can say what they like about any of those lines, but in my opinion they are far superior to Costa on any day. I did a Med. Cruise on the Concordia. Many of the passengers were as rude as some of the staff. It was a great adventure, we loved the ports, glad we went, just wish we had went on a different cruiseline. Somewhere I posted a review, Paul Motter had a few comments on it and I agreed totally with him.
I guess we've been treated to well by the other lines we've sailed to settle. If I had to choose between cruising Costa and a poke in the eye with a sharp stick, I guess I'd cruise Costa,