Re: Power requirements
"The cost of buying and operating one of these things mean that they're only going to benefit the rich; so only a very small proportion of the population and no solution to urban congestion."
I'm sure a similar argument was made against cars in the early days. "They are too expensive, so they will only benefit the rich."
Except, as more rich people buy them, more are produced, production techniques improve and economies of scale start to have an effect. Soon (although it could be decades) people on "middle" incomes get to benefit, and eventually even the "poor" can afford a second hand one.
As for the rest of your argument, andvancement of technology would eventually improve the performance and power requirements. This is also likely to be decades away, but none of your arguments are good enough to say "Why not do something usefull" to those developing these vehicles.