Well, Nuendo and Cubase (all versions) are not crashing in regular usage over here - usually Nuendo 5 or Cubase 5 is running about 70 hours a week. This might be indeed a plugin.
I sometimes (not often) have crashes (all versions) when I use for example Autotune for offline processing or sometimes when closing a project. When just recording and editing audio I can work for weeks without a hickup.
In N5.1 there are no big showstoppers, beside the bug “which can not be fixed without reprogramming the audioengine” with the disconnected VSTi metronome (you have to set it up again when you open a song).
But I did not spend much time in N5.1. I was in Cubase 5.5 for a long time beside Nuendo 4.3 - upgraded to N5 just because of N5.5. I installed 5.1 AFTER I checked out 5.5.