That plastic vs metal thing your buddy proved is the whole ballgame. Metal conducts cold right into the teeth, so once it hits the teens the slider and the coils basically weld together, and yanking just rips the pull off. I had a metal zip on a Marmot do the same thing at 11 degrees near Leadville, and what finally got me out was holding the slider against my bare stomach for about four minutes until my body heat thawed it enough to move. After that trip I started rubbing a crayon on the teeth before every cold weather run, and it worked all season. Wax or graphite from a pencil works too, just lay it on thick on the outside of the teeth. The plastic zip on a cheap tent is honestly the smarter call below freezing, metal looks tougher but it lies to you.

My buddy Rick poured a 24x30 driveway over in Saginaw last spring and the finisher talked him into fiber only, no grid, said it was "the new way." Come January he had a crack running the full width right where the culvert pipe crosses under it, and the slab had settled about a half inch on that side. Rick called the finisher back out and the guy shrugged and blamed the base, which was the same base he looked at and said was fine. Cost Rick $3,800 to tear out and repour with #3 bar on chairs at 18 inches each way. Fiber is fine for what it does but it is NOT steel, and that soft spot would have cracked with or without a bag of fibers in the mix.