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 **** 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