Rode a Nexus 8 all winter last year here in Madison, WI. Didn't have any real issues with the hub shifting (other than normal adjustments to line up yellow lines) or performance at all. I did (and do) have a ton of issues with corrosion internal to the hub that showed up in spring. Been told that the Nexus hub is notorious for this and that some of the other (read SRAM) hubs are better in that they are serviceable. I did end up buying the special Shimano lube and tore the hub apart to replace bearing balls and re-grease it. Wasn't the easiest thing in the world, but did get it back together.
Haven't been riding that bike all summer, but am starting to think about what to ride this winter and am considering an old-fashioned Sturmey-Archer... I think IGH is the way to go in winter, but am not sure the Nexus hub will make it thru. From what I have read, Shimano has addressed some of these issues, so you may have better luck...
Hope this helps...