Horses who are out all the time in bad footing tend to learn how to not break legs. The biggest risk is increased stall time and then turning out on "bad" footing, because you've got pent up mental energy taking over rational thinking and that's often when injuries occur.
One way or another, even if spreading soiled bedding in the worst spots is required, the horses here go out daily. It is their routine, and they are smart enough to go carefully. Those not barefoot are shod with studs, and rim pads.
I agree with most others here that when the footing gets dicey horses that are out all the time tend to handle it no problem. The other question I ask when people wonder if their horse is safe to turnout in the winter conditions is what does the horse have on for shoes if it is shod? I know some barns and owners that leave regular shoes on all winter long and continue to turnout on ice, snow, etc.. That is a recipe for disaster and asking for injury. If your horse is shod year round winter shoes with borium or ice studs and snow pads really are a neccessity for safety. The only time I have witnessed a horse slipping on ice was when they were not properly shod for winter.
It is "all or nothing"
A few years ago we had some REALLY bad ice in Virginia, and quite a few horses had faal accidents.
It turned out that-
Horses that were out the whole time on their regular schedule were fine. They figured out it was slippery, and were careful. (Mine were in this group, and we put down manure and shavings on top of the slipperyest areas they needed to use.)
Horses that were kept in for the WHOLE time (2 week IIRC), until the ice was gone, didn't have any ice-related injuries. They took off running when they were let out, but there was no ice to slip on.
The injuries and fatalities were primarily amoung the horses who were kept in for a week, and then let out when the ice was "starting to go", but there was still some ice. The horses, having been kept in, took off running when they were let out
My horses have always been out in any weather. I choose to let them come in/out at will. I think the risk comes when you stall them and then turn them out. They get hyper, stupid and sometimes just too crazy to be thinking. I watch mine out there where some parts of their field is a sheet of ice. They know what it is like and they react accordingly to the footing they know is there. The longer you keep them locked up inside, the more you risk an injury when they finally get out again.
If turnout is at all possible, I say do it. My guy and his whole barn were out everyday this last snow/ice and they were all very reasonable about it.
Where my retired guy is kept, all the horses were locked inside for three weeks while the ice came and went. They are all bent out of shape now. I haven't heard of any injuries luckily.