We have never sailed Carnival, so I can not fairly compare Celebrity or Royal with Carnival.
Initially, we sailed RCI with teens and young adult children. When we sailed by ourselves this was with X. We had great cruises with each cruise. Never a disappointment.
For the last, 20 yrs we have sailed with X solely, 2-3 times a year. We have chosen to sail with a verandah room, choosing deliberately NOT to sail suite, aqua or concierge class. Again, always a great cruise, with little to no disappointments.
We have sailed X 3 times since Sept 2022, post Covid. Lovely cruises with no complaints. The latest on Summit in June. No stateroom, food or service issues. Wonderful times.
We sailed May 2019 on RCI Jewel to the Mediterranean. It was wonderful and we plan on sailing on her again to Iceland/Scotland next June.
I think itinerary and particular ship is extremely important when comparing cruises or cruise lines.
We live right outside of NYC so are very accustomed to fine dining, Italian, seafood, steaks, sushi etc. We realize we are not getting a comparable when cruising and do not expect such, but again, have never been disappointed as service has always been great to help if there was an issue( there has not really ever been).
so many things are subjective, but I hope this helps, from our opinion.
have fun!