Hydrogen is neither efficient or inefficient. It is an energy carrier. Just like filthy unsustainable hydrocarbon-based fossil fuels gasoline or diesel.
How you produce usable hydrogen, and how well you use the energy stored therein, defines the efficiency of the hydrogen-powered car, generator or other device.
Unlike gas, however, if you spill hydrogen it won’t pollute your water well, cause cancer or poison you. Hydrogen can also be produced in a sustainable manner with zero emissions of carbon during production and use. No carbon dioxide (CO2) or carbon monoxide (CO).