Stability depends on how the rotors are configured. If they're articulated like helicopter rotors, an onboard computer could make automatic adjustments to blade pitch. General Dynamics needs to get this guy's phone number. I can definately see these things used by all sorts of uniformed services.
Article states that the device can't auto-rotate (like a helicopter) due to not having variable pitch.
It does, however, have veins to give some form of vectored thrust (my paraphrasing, but that's essentially what it'll be), and I suppose some roll-control might be in the cards.
Big question: Are front and rear rotors hard-connected? If they are, safer than otherwise (power lost on one or t'other and it's not a case of a multiple back-flip/forward-flip as you plummet earthwards), but only if they're independent (or semi-independent by a differential/brake kind of set-up, perhaps) can you get machine-only pitch-control. But I'm betting that a lot of the control is not by the "motorcycle handlebars with a bit of up and down" (paraphased, again) control, but body-weight shifting.
I'm also wondering if his nearly ground-level tethered tests have shown enough power to mean that flight is still possible even
without Ground Effect, because those claims for 10,000ft (or whatever they were) rely on that. Also on the ability to take enough fuel to reach that high, so even if the device could technically fly up in the rarefied air you'd need to be able to take off with enough of the stuff to last for the length of time it takes to get that high (aided by the loss of the fuel weight, it would have to be said). And one of the articles or the other so far mentioned does say it's a bit of a gas-guzzler.
You probably won't need quite the same consumption rate to make the return journey (under control!), but you still need some gas in your tank, unless that's the point you let the engine cut out and use the vehicle-mounted parachute systems and/or personal backpack type to return (at least the pilot) safely to earth.
Also, I know it mentions it as something to be done, but
more mesh over the rotor assemblies... Underside as well, even though it's blowing outwards.
(I also don't think it'd be a viable vehicle to re-enact a 911 scenario. Even with a backpack of explosives carried by the pilot, or as attached/suspended luggage, it would be no more dangerous to a building than a standard winged or autogyro microlight/ultralight with a maximum load of the similar kit, which is already possible. And if you were going for that approach, why not use a regular prop-powered civil aircraft. Memory fails me, but wasn't there a building-CFIT of a light aircraft in NY not long after 911? Not the jet that came down very soon after (again, IIRC) with it's hydraulics seized, but a small aircraft that turned into a building? My Google-Fu can't seem to extract this incident from the wider-world of air-crashes. And I keep getting info on that B25/Empire State Building incident in '45. Which was far more problematic (ended up destroying a penthouse on the opposite side of the building, with debris that passed through it). Although I don't doubt (and have ideas of my own) that there'd be far more innovative uses of such a craft, if it ever became common enough to have easy and suspicion-free availability to those that'd want to do something nefarious and intentionally infamous...)