safety is somewhat a subjective decision based upon how you "feel"... some people feel safe on a motorcycle for example.
that being said, the smart does admirably in standardized crash testing. design-wise there are no major watch-outs that should cause undue alarm...
safety has alot to do with your driving style, your skill level, the kinds of risks you take while driving, etc. in other words, safety is not a constant. it is a variable.
while comparisons to a yaris are popular, i tend to think they're not that valid in many aspects.
winter, in and of itself, does not make a car safer or less-safe. it's how to adapt to those conditions. snow tires and cautious driving should handle most winter conditions. it would seem obvious that deep snow, ice and other extreme conditions are going to be challenging for most vehicles, including the smart. under those circumstances, i dont' think the yaris holds any particular advantage.