So far most levels look like they are about at the right place from my limited experience. Different people may naturally find different levels easier or harder of course.
For me, One Way Road, and Minimalism took the longest out of the Lovely levels, both of which were largely through trying other methods that just didn't work before seeing a solution.
From quirky I found Prelude stuck out as a lot easier than the most of the rest. (btw I hope there are remakes of that level called Toccata and Finale?). Exit Stage Right and Path to Mahiman also took a while, though when I solved the latter I felt I should have saw it much sooner. And I agree, Exit Stage Right now seems relatively easy now I have found the solution!
I have done the first 4 levels of cunning, found Once You Pop You Can't Stop much easier than the other 3.
On Lix Cannon and not seeing how to do it yet, though I've only looked at it for 5 minutes or so.