To me it's the difficultly level. IMO keeping acros alive and growing is one of the most difficult things in reefing, and when I'm successful it's exciting! I had softies for decades and never even bothered with water changes. But acros, that takes a bit of skill.
Since I love acros, I'll answer the question from that perspective. There are some overrated and some underrated. I would say in general, most of the "new" corals like the previously mentioned Walt Disney are overrated. I like my Walt Disney, but I have other more common corals which I feel are much nicer to look at. Most of the older corals, such as PC Rainbow, PC Superman, ASD Rainbow Milli, Bali Green Slimer are underrated. They are absolutely beautiful corals which can be purchased for far less than some of the newer, less beautiful acros.
To me, it's all about what the acros look like (under realistic lighting). I'm happy to pay more for a more beautiful acro, with or without a fancy name.