I think you and I might have exchanged posts on this before. My point was that I believe hydrogen is a boondoggle - a waste of time and resources. The only way it could work would be tank exchange, which would require an entire new infrastructure.
The one technology breakthrough that would reduce our consumption would be to find a way to put our trains, trucks, and cars on the electric grid, at least on major routes. That would require changing to electric-drive hybrid vehicles, electrification on major routes, and some kind of trolley system for vehicles, along with a metering system to pay for usage.
Your vehicle could charge its batteries from the grid, or from its on-board generator when needed, when you were traveling away from the grid.
Hydrogen most certainly isn't for cars. But as an energy transfer system could it be more efficient than electricity? A fuel cell in every house? A heating plant that exhausts water vapor and nothing else? A distribution system that for the most part exists as natural gas lines that don't lose the huge percentage in transmission that electricity does? An energy transfer system that can have the production located near the energy source i.e., hydroelectric, solar or atomic? An energy transfer system that can be stored as opposed to electricity that must have expensive peak demand capacity?
Drawbacks? The initial energy transfer if you start with water can't be very good since half the energy goes into free oxygen. If you start with natural gas - well what would be the point?