I don't like things around my neck in general. I have a Becker Necker and I could see wearing it around my neck in a survival situation, but I share the same concerns of falling on the thing and getting slammed in the sternum, etc. The Kydex sheath on the Necker is tight and I have little fear of the knife falling out. You could add a strap with a snap like a pistol holster, a Velcro band of some sort, or even an inner tube rubber band to keep a knife in the sheath. That defeats any quick access, which is the whole point of using a neck carry, IMHO. If you have a too-loose Kydex sheath, it could be heated and re-worked.
For the Necker, I added a Tek Lok and found that a 45 degree/cross draw belt mount worked best for me. This is for hiking/wilderness carry. It would fit in a pants cargo pocket too. If you don't mind having your knife tied to your pack, most neck knives would mount nicely on your pack shoulder straps.
I use beaded chain threaded through silicone aquarium air tubing to make my survival necklaces. Break-away is a concern for me. I hike in areas with a lot of brush, so the chances of garotting myself in a fall are real. I have made some rigs using paracord with eyes tied in each end and a small loop of chain to form a "weak link" break away.
I've also used a 1/2" long piece of silicone tubing and puled both ends through it and then flamed the ends of the cord to form a small ball on the ends so they won't slip back through the tubing easily. My hope is that the tubing would break or the cord would pull through if I got hung up. You can use heat shrink tubing and braided cord too, just making a simple butt splice with the heat shrink.
My preferred way to carry survival necklace items (compass, firestrarter, knife, micro light and whistle) is to put them on a cord lanyard, loop the cord around my belt in a lark's head and drop it all in my pants pocket. That is secure and quiet and there's no safety issue.
The only reason I could see for neck carry is for self-defense/secuirty and my local city carry laws don't allow fixed blades, so I haven't gone that route. I would be concerned about getting throttled with my own rig if it were around my neck, so the "shoulder holster" arrangment makes a lot of sense to me. I don't think it would work well with a pack though.