Greg...
The Poco is coming solely from experience.
Underground distribution systems have fault rates trivially low compared to overhead distribution.
That gap has been industry knowlege for generations. It's the primary reason that EUSERC pushes for underground distribution even in residential areas.
When this is done from the very beginning it's quite practical -- going back fifty years.
These days, horizontal boring rigs are making it possible to retro-fit underground distribution in many older neighborhoods.
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As you might expect, weather just does not effect underground circuits.
The one area that I know that has issues is Las Vegas. The ground heat is so high there that the rules shift. Local rules generally prohibit underslab circuits -- limiting them to the shortest paths possible for grocery stores and the like. Not uncommonly, the conductors have to be severely de-rated. (#8 being required for 20A circuits and the like -- everything being re-engineered for Las Vegas conditions)
