Anecdotal, but I've ridden a bike in winter only once. There was a lot of melted snow "slush". I almost slipped twice. One of those two times, I was right in front of a car when my back wheel slid off to the side, and I caught myself from falling just in time. I've never done it again, it seems suicidal to me. It was a mountain-style bike, and I would have assumed the tires had enough grip. I'm sure you can pick up tricks and get better at this with experience, but it seems at least an order of magnitude more dangerous than summer time biking.

Studded bike tires are common in bike friendly, cold cities. Work great with ice. It's all about the right tool for the job.

Although what a lot of winter bike commuters do in my city is go by bike 95% of the winter, and take the bus when the conditions are really bad.

I biked every day of the year for about ten years in NYC with an average commute of about six miles each way. This included a ride over either Manhattan or Williamsburg bridge, which get icy faster and stay slushy longer than terrestrial roads.

Always on regular 23c or 25c road tires.

I usually fell over about once per winter, which always kind of sucked. But by my estimation, riding the subway was a more miserable kind of experience.

You slide around a bit but you get used to it. The bike is usually but not always more stable than it feels.

It really seems to be an activity that takes a special type of person to enjoy - my wife, being an avid summer biker, tried winter biking a single time, and came back with a giant bruise on her knee and a bent pedal and swore never again.