A car has access to all 4 wheel sensors independently, it can apply brakes on each 4 wheels independently. It can always turn the steering wheel in the right direction, and it wouldn't panic.
Also, it would always drive the 'right' speed limit... sure, other human drivers might get annoyed, but assuming the true 'full self driving' future happens, there shouldn't be many of them on the road in time anyways.
What does that mean? That it actually is solved by humans? Or that it's not, and we just take the increased risk? If the former, then we can automated it. If the latter, then the self driving cars can also drive at increased risk, opt-in of course.
If we built planes that were only as safe as highway driving, people would be outraged.
We aren't exactly rational about this stuff, and we expect a lot more from machines we don't directly control.
But I like your opt in solution. Maybe the UI loudly complains about the risk of current conditions and sticks to <5 MPH, unless the user enables "never tell me the odds" mode.
Driving safely on frozen surfaces is not a solved problem for human drivers, but most of us insist it is a perfectly reasonable thing to do.
If the vehicle drives as slowly as it should in those conditions, it would probably frustrate a lot of people who really depend on their false sense of invincibility.