I use flat pedals with my fat bike in winter with winter boots on really really cold days or La Sportiva Winter Trail Runners (they are warm and go above the ankle). After a winter of training mostly like this on my fat bike with a few trainer rides mixed in, I did a March olympic distance triathlon and had one of my best bike splits ever. My fitness did not suffer from riding flat pedals. Actually felt like taking the dumbbell off of a bat going to the tri bike with clipless pedals. As a bonus, the La Sportiva’s work great for snowshoe running or just running on the snow.