This could still be a problem. Human drivers are still available on these days today, and if services like Waymo take over and theres less Uber (et al) drivers, then we'll have no support on the worst days.
My understanding is Uber etc. charge and pay based on demand, so if that means the (fewer) human drivers are that much more expensive on snowy days then so be it. If it's a blizzard and Uber fares work out to $100/h I'd be very surprised if drivers couldn't be found.
Basically if the planes are flying the robotaxis will need to work.