Arg cought by the summary.
My point is that the absolute maximum flighttime for electric UAVs is less than one hour, in a lab, draining the battery to death(literary)
First, UAV is a mighty broad brush. An inexpensive electric glider with a motor on it and big battery can fly for literally hours -- without even using thermals or other lift, and they can increase it further -- and the battery will be just fine when you land, just recharge it.
Now, this is a multicopter -- far less efficient. Still, with good batteries, keeping the weight down as much as possible, and just sitting there (using as little power as possible) -- 50 minutes doesn't surprise me at all. And the battery will be ready to fly again after recharging it. (Though it's possible that they used non-rechargable batteries to last even longer for that specific test.)
In practice electric UAVs have a flight time of 12-25 minutes depending on payload. Saying it will fly 50 minutes is just wrong.
Most of the hobbyist multicopter models have flight times like that, yes -- we want something that's lively and fun to fly, and don't just want to hover there. But we could make something that lasts 50 minutes if we wanted to and didn't mind spending some money.