Dangerous if you're not dressed for it.

Wind chill doesn't matter much if your skin is covered and you have wind proof gear on. -25 C is about -13 F and I typically ride in those temps (and colder) multiple times a year. This year has been milder.

I do live in a city though and not out in the country. Even then if you have a functioning cell phone a mechanical problem is probably not life and death, - again, assuming you're dressed for it.

I've had a free wheel quit working on me in cold weather on one occasion. It wasn't even the coldest day I used it on and it happened after I stopped at a convenience store a few blocks before I got to work. It was working fine before that.

Not sure why that freewheel at that time and never any others.