The crank slipping is the back tire slipping. At 6" your pedals hit the snow. That's very difficult to keep going. I did about a mile once. I had to stop and rest a few times. The wide tires of fat bikes under some snow conditions are harder to push through the deep snow because they are wider, like 5 inches of soft snow on pavement. In conditions like that I can ride a lot farther on my 2.4" tires, than 3" or 4" tires.
No one rides a bicycle in 12" of snow. In 5" of snow there are often deeper drifts to stop you.
Been riding on ice as snow as much as possible starting in 1968 on motorcycles, and 1995-6 on Mountain bikes.

The conditions here today allow my little dog to walk on the crust without sinking in. While I beak through every step. Good exercise. Soft snow today with a hard crust is too much work to ride.