The cheesy ones with plain steel studs, yes, it's A Bad Thing. Good ones with carbide studs, no problem. Nokians use carbide studs. I'm entering my fifth winter on a set of W106s.

That said, they're tough to pedal around compared to road tires. I used to have second set of wheels for mine, but the expense of a dynamo hub put an end to that. Now I use a second bike for the clear and dry roads days. Fenders and Kool Stop salmons are its only concessions to winter.

I too have never flatted in winter. I also attribute this to the nice, thick tread of the Nokians, and the plows pushing debris to the side along with the snow.