I suspect that Waymo will do better compared to a human outside of Phoenix than inside it.
The hardest thing that Waymo will have to do is to interact with humans behaving oddly. Phoenix has those, just not as many as NYC/LA/SF. But one thing computers do exceedingly well is scale. Dealing with one strange human is very hard for a computer, but dealing with two or more of them is very similar.
Snow / ice / rain on the roads means changing underlying models, behaving more cautiously. Computers can switch modalities much quicker/easier than humans.
Fog and other visibility problems are sensor issues. Humans are stuck with the old Mark 1 eyeball, but self driving cars have a mix of sensors. These sensors may currently be worse than eyeballs in fog, but may not be in the future.
Even given those advantages, you still start in an easy location. Going from 0 to 1 is really hard, give yourself all the advantages you can. But going from 1 to N is a lot easier.
Fwiw your conclusion is not really supported by the rest of your statement. Just because scaling from 1 to n is easy doesn't make scaling from zero to 1 feasible. And even if it is (feasible) there's no guarantee they'll be able to do it soon.
The hardest thing that Waymo will have to do is to interact with humans behaving oddly. Phoenix has those, just not as many as NYC/LA/SF. But one thing computers do exceedingly well is scale. Dealing with one strange human is very hard for a computer, but dealing with two or more of them is very similar.
Snow / ice / rain on the roads means changing underlying models, behaving more cautiously. Computers can switch modalities much quicker/easier than humans.
Fog and other visibility problems are sensor issues. Humans are stuck with the old Mark 1 eyeball, but self driving cars have a mix of sensors. These sensors may currently be worse than eyeballs in fog, but may not be in the future.
Even given those advantages, you still start in an easy location. Going from 0 to 1 is really hard, give yourself all the advantages you can. But going from 1 to N is a lot easier.