If your new mac ships with a Retina display, then please give up using Nuendo 5.5 unless you have to deal with 32bit plugins.
(Nuendo 7 supports 32bit plugins, and you can run Nuendo 7 with newer Nuendo licenses like Nuendo 8, etc.)
Also, patch your CoreAudio bundle, especially if you are using class-compliant devices:
https://www.steinberg.net/en/support/knowledgebase_new/show_details/kb_show/workaround-using-older-steinberg-software-under-mac-os-109-mavericks
P.S.: I suggest you to use the latest version of Nuendo because it is sufficiently optimized for new macOS.