Capacitors
My main concern with these is safety. I don't know how badly you'd have to damage a giant capacitor to make it unleash the Wrath of Thor in a single flash of lightning. I assume the inventors are working on that, and when it comes down to it, it's not much different than a shorted battery catching fire, or a pool of gasoline near a spark.
By definition, any form of highly-condensed power is going to have some danger to it.