There are other considerations at work here as well. Having a 70l/d ratio means huge wings on something that will be this heavy. Gliders have big wings for their size and they have a very very low gross weight (ASK 21 2 seater at 900kg and a 17m wingspan). On top of just being absolutely massive, having all that wing area can cause problems in other than optimal weather situations. An ultra low wing loading will mean that the craft responds like a kite to any air movement so it would be very uncomfortable for the passengers who just want to get where they're going.
I agree that the chances of power loss are remote, especially compared to the current tech in use, but especially with new designs, you need a way of handling it. The issue isn't so much any individual motor but the batteries and support systems. They'll need a cooling system most likely and any failure will turn the whole thing into a Note 7. Also, sorry about using LiPo as a generic term. LiS batteries will definitely help the situation somewhat.
You refer to L/D ratio and lack of wing loading as if it's the same thing but it isn't. High performance gliders are generally ballasted (sometimes about half the weight is ballast) in order to increase the wing loading and increase flight speed. Our aircraft would have a 300km/h cruise speed, and so will similarly have a fairly high wing loading, even if it does need long, high aspect-ratio wings. This is helped by the fact that the sort of high performance lithium-ion batteries that you'd be likely to use (i.e. like Tesla uses) would be a good twice as dense as fuel. This increases your effective wing loading for the same outer mold line, allowing higher speed efficient cruise at the same altitude.
As far as battery and support system being a central failure point:
One interesting thing with electric propulsion is it's fairly easy to split the batteries and subsystems up /as well/ as the motors. So each motor pod could have its own small battery nearby for (possibly emergency) take-off and landing (this could also help reduce cabling mass, especially for the high currents you're likely to need for the vertical takeoff and landing portions). The cruise batteries could remain centralized, since you'll need a lot more of them and mass efficiency will be critical (and you could rely on your very good glide ratio).
300km is probably optimistic for this prototype. Wings are pretty short. But with extremely good lift to drag ratio (as seen in gliders, the best of which can get over 70 lift to drag), you could definitely get even 1000km range even with existing battery tech.
Range = (ratio of battery mass used for horizontal flight to total mass) * (lift to drag ratio) * (efficiency of propulsion system) * (specific energy of batteries)/(acceleration due to gravity)
So, if cruise-batteries are 0.5 of the total mass, lift to drag is 50, propulsion system is 0.75 efficient (75%), battery specific energy is 1MJ/kg (or 10^6 m^2/s^2 in alternate units, about 300Wh/kg, as good as very best lab-scale lithium ion batteries... lithium-sulfur is better), and gravity is 10m/s^2 (rounded), you have a range of:
0.5 * 50 * .75 * 1(MJ/kg)/(10m/s^2) = 0.5 * 50 * .75 * 10^6(m^2/s^2)/(10m/s^2) = 1875km.
Of course, 50% useful cruise battery weight is probably optimistic (although not too different from long distance airliners whose take-off mass can be roughly half fuel) and a lot of weight will be needed for the vertical take off and landing motors plus the payload, but it does show you what's possible even with existing battery tech.
I tend to think the prototype they showed probably won't get 300km range. Probably need longer wings for that. But their eventual goal is achievable. By the way, for long range, they're not likely using LiPo but lithium ion, perhaps those ubiquitous 18650 cells that Panasonic makes (of Tesla fame). And greater range is possible, especially as lithium-sulfur batteries start becoming more widely available (to speak nothing of lithium air).
As far as losing power to an engine, well, there are lots of engines. Also, electric motors and batteries are very simple and can be built to have extremely high reliability. The batteries and propulsion system can be built to be totally distributed, in which case there's really no feasible scenario where power is lost (unless control is lost, in which case you'd be screwed in a conventional airplane or helicopter, too). Additionally, a ballistic parachute can be and is used.
I don't think this flight is a breakthrough. A decent team could do the same thing they just did, and lots of people (including at NASA Langley) have been working on the same goal. Teaming a bunch of electric motors capable of tilting is a thing that several groups are pursuing, lots of people have done at smaller scale already, and it's only a matter of time before it's done at a large scale as well.