#ergo: they're unsafe for serious transportation purposes

[ceescedasticity]

I'm not sure they should be in the street, either — all the problems of a self-driving car but more likely to be run into accidentally.

The problems being highlighted in this post, at least, seem to be less due to the 'motorized' part and more due to the 'bulky object with no consideration for other's needs or behavioral flexibility'. You could get the same problems from a very rude person with a heavily loaded shopping cart. Those just aren't common.

Not that the motorization isn't also a problem — speeding things on sidewalks are dangerous. You also get that from bicyclists but at least those have humans steering them.

In conclusion I'm not sure anything should be on the sidewalk or the street without live supervision in case of unforeseen situations requiring judgment calls.

Edit: Without being capable of observing and communicating with others, exercising judgment, and modifying its behavior accordingly — even in foreseen situations.

Edit 2: I would venture to guess that we wouldn't be very happy if the sidewalks were suddenly full of delivery people with heavy handcarts, either. Is this transportation space for people or for cargo?