In Nuendo 8.0.15, Undo doesn’t working when using the Sample Editor. You have to open the History tab and go back to an earlier time in order to undo. You can use Undo normally everywhere else. However, even when you leave the sample editor window, you cannot undo anything you did while working there. I’ve had this happen multiple times now, and every time I go to Cubase 9.0.30 and it works fine.
I love Nuendo 8.0.30 though. Very stable for me here.