Seems like it’s really only suited for cities. That’s not where the challenge is.
There's lots of challenges with mail delivery depending on the area, density, business/residential ratio, and many other things. Those vehicles satisfy some situation. "the challenge" is not a thing.
Only later that I discovered I was the dumb one.
IDK how you can possibly economically send mail to places like foreign barely habited deep pacific islands for the same price as mailing to your neighbor.
China does that, for example - which is why you can get a $3 widget on AliExpress sent from China with free shipping.
However, right now congress doesn't act like they're doing that intentionally. That doesn't seem to be the policy they're aiming for.