I also use both steel and soft shackles for the bumper mounts. I wouldn't bet on the soft shackle failing first, BUT I would bet that the 2-shackle combo will handle MUCH higher loads than just a soft shackle connected to a deburred D-ring mount.
Most examples I've seen posted aren't a big enough diameter to maintain full strength. Minimum full-strength static-mount ratio for dyneema is 1:1, so a 1/2" soft shackle needs at least a 1/2" diameter bend. The standard 3/4" steel shackle gives 1.5:1, which is perfect.
My shackle mounts are 3/4" thick, so a proper grind is at least 1/4" radius on each side leaving no more than 1/4" flat in the middle.
People get away with less because shackles are far stronger than needed. But you really don't know how much strength you're losing with a grossly-undersized bend. It'd be cool if someone did some testing.
I also have tow hooks on my front bumper, which have a nice radius and super convenient for attaching soft loops.