Deep snow is much harder to ride in than most people think, about 5 - 6" makes for much greater effort and very squirrely handling (studded tires help handling, but don't make pedalling any easier).
I am a platform person - I can wear different footwear depending on temperature and can walk if the bike freezes up (didn't happen last winter). The 3 or 4 times I hit black ice and fell, I don't think clipless or platforms would have made any difference, but on rough, slushy ice/snow covered pavement, I occasionally get a brief feeling of security by lifting a foot off the pedal of the side I think I'm going to fall on.
Snow depth is probably less of a worry than most people think. The urban roads around here are usually well plowed and salted, preventing much snow accumulation on the travelled portion of the road. One consideration during winter is that "the travelled portion" of the road gets much narrower due to snow banks and unplowed areas.