They work (most of the time) in Phoenix and SF because they've mapped every single inch of the cities by now and there are no adverse conditions. It's not scalable.

Why is that not scalable? Mapping out two large cities for an experimental project in a few years seems scalable, expand to new cities over time with additional resources.

I suppose it is, but not in a Silicon Valley way. They could scale to "large Southwestern city taxi service," but it wouldn't earn back the investment or deliver on the hype. If that becomes the ceiling I bet Google will simply shut Waymo down.

If they work out how to deal with, say, New York weather conditions, there's potential, but they don't seem to be any closer.

I think you’re conflating doable and scalable.

Or perhaps my threshhold for “scalable” takes different parameters and weigh these inputs differentfly from you.