It not only travels faster (it has to, for lift, or the fuel consumption for hovering goes through the roof, and there goes the cheapness), but has in addition the gravitational energy to get rid of when it goes wrong. From just 400 feet up, it will crash at at least 100mph.

Failure of imagination, based on postulating the currently existing means of propulsion (rocket or jet engines). Here are some zero energy (but progressively harder) alternatives: buoyant force, magnetic hovering, gravitational repulsion. Or consult your favorite hard sci-fi. Though I agree, finding an alternative to jet/prop/rocket propulsion is the main issue.

When it crashes, it could crash on anything. Nobody is safe from skycars.

If it doesn't have to fall when there is an engine malfunction, it does not have to crash.

Controlling an aircraft is also far more difficult than controlling a car, taking far more training, partly because the task is inherently more complicated, and partly because the risks of a mistake are so much greater.

This is actually an easy problem. Most current planes use fly-by-wire, and newer fighter planes are computer-assisted already, since they are otherwise unstable. Even now it is possible to limit the user input to "car, get me there". Learning to fly planes or drive cars will soon enough be limited to niche occupations, like racing horses.

Incidentally, computer control will also take care of the pilot/driver errors, making fender-benders and mid-air collisions a thing of the past.

Optimistically, I can't see the Moller skycars or anything like them ever being more than a niche within general aviation.

Absolutely, this is a dead end.