If a single engine on a quadcopter loses power, it crashes [2]. There is work being done on mitigating this, but I don't see why you'd go for a quad versus tri- or helicopter.
(Quadcopters are better than other aerial platforms at one thing: maneuverability. To get this agility they sacrifice speed, efficiency and payload weight.)
[1] https://en.wikipedia.org/wiki/Autorotation
[2] https://www.quora.com/Can-a-quadcopter-fly-without-one-rotor
You're wrong about a quadcopter not being able to fly with 3 rotors, see this video: https://www.youtube.com/watch?v=bsHryqnvyYA
That looks like it would be quite a ride with a human on-board, but would probably suck less than crashing.
If one motor fails, the quadcopter could turn off the opposite motor and reverse the direction of one of the remaining motors, so that it is left with two opposing motors spinning in opposite direction. The torque of two motors might be enough to keep it flying. Some steering would be possible with the assistance of the third working motor which could spin in either direction as needed.
Pretty sure you won't survive that transition since your craft will be completely out of control in the seconds it takes to kill all rotational velocity and spin a rotor up again, at least for large sized rotors.
There's enough open source information laying around on the internet to figure out the power draw of the Ehang in flight and from there you can figure out roughly how much the batteries on it weigh (spoiler: it's many 10s of kilograms for the 23min flight time they claim [1]).
Don't forget that these batteries will degrade with time, especially in such a high-load performance regime. This will further limit your flight time until you replace them. On top of that, having 8 tiny props is always going to be less efficient than a fixed-wing or helicopter-like design.
If you're a Chinese billionaire, you don't care that your "flying car" has these limitations, but these same limitations (among many others) preclude the Ehang from becoming an actual practical method of transportation.