Anything made out of quality horsehide or cowhide with properly sewn or solid loops will be virtually impossible to rip off of someone in an entangled fight. Leather stretches, but does not tear or break unless it is old, dry, and thin.
Think about it for a second - do leather belts fail because they break or because they stretch out and get floppy?
I would not be concerned using a Kramer, Red Nichols, Milt Sparks, or even Don Hume holster in FOF. I'm more concerned about leather holsters that mount plastic or metal clips using a screw by way of retention. I have a very nice Alessi Talon Plus with a plastic clip that is sewn into a pouch on the holster. It works great for and locks under the belt firmly, but I'm not convinced it is as sturdy as a metal clip or would hold up to a weekend long ECQC class. Which is why it is basically the holster for quick dog walks or trips to the stop and rob.