Hi pmtm,
as often the answer lies between, some functions of Nuendo helped me in my work, others not or better said some important functions are still missing or would need a improvement. For PostPro, Nuendo is the better choice as it has some specialized tools for that segment, e.g. improved surround functions, sound design tools, the ADR function (also great for dialogue work in games), the import functions etc. The 3D and VR sound features are exclusive to Nuendo but very special as the market for this is not easy to reach because of big investments (tech and know-how) and the market is not the biggest. For Game Audio Nuendo is over Cubase as only Nuendo delivers the direct exchange with wwise (if you need it depends on the developer studio you work with, often they only need the wav files), better batch processing functions and has some sound design tools on top. For composing Cubase and Nuendo are mainly the same and have a good set of tools. For film scoring both would need some improvements for e.g. more comfortable tempo and hit point adjustements, score sheets with cue points, renovated and improved chord assistant,…
By the way - i did not change the whole field of work, i just added post pro and games audio 
Another help for deciding if you need the crossgrade would be installing the test version of Nuendo, worked parallel with Cubase at my test and helped me to decide. So you could get a better idea of the additional feature set.
Greetings.