[Flieger]

I didn't watch the videos, but the disadvantage to capacitors is that their voltage drops off continuously- steeply at first and levelling off near depletion, exponential decay or whatever you want to call it. Batteries are much better about maintaining a relatively constant voltage for a good portion of their charge. Automotive electrical systems need very specific voltage- more crucial on new cars. So you need some more complex power electronics to manage the voltage output from the caps. They aren't a drop-in replacement.