The bottom line in all of this is anyone can have an episode of what they perceive to be poor customer service, and with any company. People google "poor customer service Carnival Cruise" or words to that effect, and they see a lot of search hits and think "this cruise line is messed up". But when you consider the tens of thousands of people who cruise with Carnival every week, the track record of complaints is actually minimal and I'd wager less than most other companies of similar size.
I've never been a believer in "the customer is always right". That is frankly a crock. Customers may not hear the answer they want, and so they say they got poor customer service.
Sure, some complaints are valid. Everyone has a bad day and the cruise lines don't like it when somebody goes away mad. Those messages you hear about calls being monitored to ensure quality are not just blowing smoke. But they also know you can't please everyone and they have to make business decisions as to when they draw a line and say "I'm sorry but that is all we will do".
Unfortunately, for many all it takes is one occasion and they take their money elsewhere. That can be a risky venture in the cruise business because you can easily find yourself fresh out of cruise lines after a while. Because one thing is certain, and that is that all the cruise lines get complaints and not all of them are resolved to the customer's satisfaction.