Maybe I'm totally off with that statement. :)
Like I said that's just the impression I've got from a lot of lurking (and reading) online.
With regard to PVE, I don't think the impression that we're at the more difficult end of the scale is a false one necessarily. I can't speak to PVP since my efforts there have been anywhere from pathetic to catastrophic :)
Playing a shadow priest at an average level (whatever that means but work with me here) is not overly difficult. However, playing one really well and consistently squeezing out that last 10%-15% of your theoretical damage is a lot harder than it used to be.
The devs promised spriests burstiness at Blizzcon (and many scratched their heads and laughed a lot) and have largely delivered on that (and still are). When to be bursty and when to be more traditional with damage over time is now a factor. It's two pretty different play styles.
There are a bunch of situational abilities to think about:
As mentioned above, we're not just strictly about dots now; we have to watch for target low health burn phases because we do something different there; there's RNG associated with proccing of orbs and its relationship to Empowered Shadows; Archangel for those who have it; Shadowfiend; the list goes on. In a 10-person group, we may be called upon to drop a shield on someone or toss out a Hymn. CC abilities are not as strong as other classes but MC is always an option. Silence is sometimes useful. AOE is back.
That much flexibility has its perks but it has a downside of complicating things a bit. I'm running my main with two shadow builds, one for max DPS and the other with control abilities just so I can switch between fights if necessary.
During a long fight there's a lot of things to juggle on top of multi-dotting. Having to move during all of this is just another thing to deal with.
None of this is to complain, however. It's all part of what makes shadow priesting so damn interesting.
That said, it's fair to say that everything (damage, tanking, healing) has gotten harder in Cataclysm for everyone. Class rotations are pretty much a thing of the past; everyone has priority systems now and a variety of situational abilities. And many think the game is more difficult; some a lot more difficult (not to mention dramatic rewrites of class abilities every so often).
EDIT: For clarity and because I just can't seem to get it exactly right the first time. Edited, May 21st 2011 1:16pm by Moanique