Nuendo? I don't know as I have never used it.
IMO Wavelab is the deepest, most powerful, and flexible, mastering environment available. This also means it's not the quickest or easiest to use.
There are many features not found in other programs, and I have found nothing that comes even remotely close to the error detection and global analysis functions found in Wavelab.
The frequency 3D analysis is the best I have ever seen when looking at any wave file in the frequency domain. You can look at it from any position in color.
As mentioned above the batch processing and batch renaming functions are very nice. You decide if you want to spend hours converting hundreds of .wav to MP3 or minutes. And that is very basic batch processing. You can insert dozens of other functions while converting too. Some of these functions can be very involved with lots of processing steps.
Smart bypass (depending on your console set-up) has saved me hours with clients who experience loudness bias when doing A vs B comparisons. This eliminates any volume bias and allows the listener to focus on the actual mastering changes.
The Spectrum editor has become useful on some occasions when nothing else works as well.
For basic or general editing, Cubase is fine. But it's not nearly as extensive or complete as Wavelab which is a dedicated editing/mastering environment.
The Audio Montage is made for mastering. With start stop CD markers almost anything is possible when creating CD's.
All this takes time. You don't learn it over night. But when you have questions not answered by others, PG, the creator of Wavelab will always reply within a day. That is great technical support.
1. Effect Morphing-This is NOT an answer to automation, but is as close as you can get in Wavelab. In other words, forget about using the R & W buttons found in Cubase automation.
2. Lack of a decent manual. Unlike WL6 and prior versions, there is no manual. Not a printed manual or a pdf manual. The WL7 manual is completely lame and often not any help. If Steinberg doesn't change their mind about this I think in time Wavelab will die.
3. The Wavelab-Cubase integration was lost many years ago because of the introduction of multiple undo's in Cubase. I have missed that very much, but as time goes on, most users don't even know what I'm referring to.
So only you know if you "need" it.