I remember hearing about the complaints with Fallkniven sheaths before, but Ive never owned one so I cant comment on it. Camp & hunting knives need to do things like whittling, some light chopping & the edge needs to stand up to hitting bone and the occasional pebble or knot, so it shouldn't be too fragile, anything much over .5mm is usually overkill tho. They often make the stock on modern "survival" knives too thick, 3-3,5mm is perfectly fine, 4mm for larger knives. But im not an outdoorsman or hunter so my experience is limited.
I think a lot of this might have to do with companies protecting against warranty claims & returns because of damaged edges or tips. Lionsteel has gathered some infamy for that, dunno if its gotten any better. What I can say is that my thrill has 3mm stock & is .5 behind the bevel. Way too thick for a slipjoint, or any folder really (unless it says "cold steel" on it
), but its become the industry standard.