If a neck knife happens to slip out of its sheath as you are bending over, you are in deep deep trouble. I've bought a few, and after a couple near misses, have sworn them off. And unless you have a heavy sweater or hoody on, they are rather obvious, even the flattest varieties. If I had to pick the "winner" of what Ive tried, it would be a tie between the Buck Hartsook or Kaala, depending on the size of knife you had in mind.
I read on one forum a while back where a guy almost killed himself with a Cold Steel Urban Dart in a neck sheath. Slipped out as he was going prone and stabbed him pretty deep, a couple inches more and he would've hit organs.