Though I like the idea of neck knives, I've had two concerns--the first being the aforementioned get-hung-up-and-decapitated issue. (On other threads, I've advocated referring to neck-knife rigs by the name of that Saddam Hussein cousin who accidentally got decapitated when the hangman miscalculated the drop--I forget the guy's name now.) Sure--not the sort of thing that'd happen frequently (or to any individual more than once)--but I've done enough outdoor recreation to know that you never know what to expect (which is why I usually carry a knife to begin with), and if I fall down a slope or out of a tree next time I'm out, it won't be the first time.
One solution I've used for that problem is to run the "neck" loop not just around my neck, but over one shoulder so that the knife hangs under the opposite arm. May get sweated on more there, but at least I know it's not going to pull my head off if it catches on a branch or rock or rear-view mirror or whatever.
Another issue, of course, is what happens if the retention device fails. With a knife carried point-down on the belt, if the snap fails, odds are the knife will stay in its sheath, and will fall past your leg and onto the trail if it doesn't. Your foot might be at risk, but that's about it. If it's point-up under your shirt, however, you could easily find yourself with a razor-sharp blade unexpectedly skittering around in the close environment of your heart, lung cavity, intestines, kidneys, and various other things that it might be inconvenient to get sewn up if you're 30 miles from the nearest blacktop. No offense meant to neck-knife aficionados, but my take on it is that it's an iffy risk for the benefit.