People tend to forget that high quantities of stored energy have an inherent danger. Laptops catching fire because of lithium-ion battery failures are usually the only hazard that people tend to remember. To truly have a consumer safe device you want something that can be charged quickly but the maximum discharge rate is closer to a conventional battery.
The point is that when you exceed a certain speed of energy release the device starts resembling a bomb more then a battery. A wrench dropped across both posts of a car battery is spectacular enough in a very scary way (even before the battery actually explodes.) (And don't try this! An explosion can spray acid a LONG way.) A device that can discharge almost instantly is even more destructive. We obviously need to be able to store energy for so many different reasons but the method needs to remain safe even when handled in a completely negligent manner... like a consumer device.