Carnival is basically the bottom of the barrel in the cruise ship industry, cheap crap for the masses and they maintain this cheapness by poor working conditions, slapdash ship design (count how many ships caught fire in the past few years, almost all Carnival) and low quality amenities. Like no frills airlines, there is of course a market for cheap, but you get what you pay for. Even on much better lines it isn't an easy job, but my friends in the cruise industry note that it is a decent living for people from poorer countries, and can be fun if you land on the right ship/line. You do work 7 days a week but the commute is short, and on decent ships your life is not really that bad.
But as the article pointed out, that choice of where you work can make a huge difference if anything goes wrong like an injury, and that's where none of the lines are really all that great. Even there while there are always new people they can hire, word does get out who to work for, and if your rep takes a fall because you treat people like crap, you won't be able to hire good people, and the guests will see the quality drop to point where revenue gets hit. Cruise ships are enormously expensive to run, and competition is strong. But my family has been on enough cruises to know who we would rather be on, and how decently they treat the employees is a good indication of whether you should spend a little more and get a better experience.