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.
Don't hear many anecdotes about FSD in Pittsburgh on a night in January...