I've got a Nexus 7-speed hub and find it works great in the winter. No issues because of the cold.

I've had it for 4.5 years and ~6,500 miles and live in Cleveland. I put on studded tires in the winter and ride through the winter months. I don't ride below ~5 degree F (glasses fog too much), but the shifting works fine down to that point.

The best part of the IGH, for me, is that the chain has a proper chainguard and snow and ice don't accumulate.