Up to a certain degree they work, as humans can drive in fog or heavy rain/snow as well. If visibility is so bad that a human wouldn't be able to drive, I wouldn't want to sit in a self-driving car either, no matter if it does use vision only or has additional sensors.

Any more or less than the human equivalent?

I'm not following the news, but I haven't seen any videos set in what Canada looks like 4 months per year.