I would assume that when liveaquaria says easy versus difficult it is referring to things such as feedings. For example, a mandarin is listed as difficult because of their feeding requirements and general dependency on
pods & mature tank. It is true that the Blue Tang is easy to take care of, but liveaquaria does still acknowledge that this fish is more susceptible to many fish diseases. In my experience, although I am still new in the hobby, their information is credible & reliable and is a good starting point. But, as with any animal, one fish may not fit the general description...so a peaceful fish may be very territorial, or may be a picky eater.
HTH