Hydrogen combustion converts oxygen into water steam, a far more potent greenhouse gas than CO2. Also it converts atmospheric oxygen into water that is not easily converted in nature back into oxygen so we would slowly deplete our own oxygen. CO2 on the other hand gets turned back into oxygen by plants and higher CO2 levels help plants grow even faster. If they are making hydrogen by splitting water molecules you are obviously going to also create oxygen to replenish the air butt it take more energy to split it than you gain from burning it.

The problem with natural gas (methane) is it needs a lot of energy to keep it cold enough to be a liquid, if you store it as a gas, even at extremely high pressure, you will burn it up very quickly, or in other words a very short driving range. Seems to me the best way to use the methane is to finger out a way to dissolve it into gasoline somehow.