OK, watched the video completely. They are already using computer-controlled gyros and are not likely being controlled by humans, but by software. Like AJD wrote, the limit is one of the craft's ability to make headway into the wind, inertia, and drag. IMO, those would likely be good in winds up to 10mph +/-. Also, these things' flight times would be measured in minutes, as in less than 10, more likely less than 5.
Some of my RC aircraft are lighter than these. I fly the fixed wing craft without gyro assistance in 5-10mph wind. My little coax heli can not handle more than a 2mph breeze, though. Just not enough grunt and too much cross-sectional area.
Now if they could get these things to identify and track human heat signatures, or make effective use of facial recognition technology, you would have one hell of an antipersonnel device. Scary yet intriguing.
A common term for such a system is "killer bees." Usually work in numbers, rather than singly.