There are certainly cases where NX works better than VNC: Say, with well-behaved applications on very slow connections. But there are also many cases when VNC outperforms NX: I don't think NX can at all achieve, say, the same framerate for OpenGL applications on a fast network. TurboVNC/TigerVNC (or the Sun or ThinLinc versions of it) wins here.
For many (or even "most" in my context) customers/cases, the VNC bandwidth requirement is no problem. I can say this after delivering VNC based solutions to customers for the last 7 seven years.