To me, there are two aspects of neck knives that need to be considered :
weight and size.
Even if weight is kept down to a bare minimum, the knives are often 4 inches or more, which makes it hard to let your arms rest on your stomach without coming in contact with the knife.
I have a couple of neck knives that I find ok though - Fällkniven G1 (a bit long) and Fällkniven WM1, which is ok.