Making H2 is no problem, although doing it in an energy efficient fashion (necessary for a good energy carrier) is not so simple. Storing it in stationary applications is non-trivial, but a solved problem. Storing it (in a practical way) for mobile applications is very hard (bulky insulation, and you have to deal with vented hydrogen). Storing it safely
(i.e. American-consumer-proof, which is much more stringent than merely fool-proof) for mobile applications is very hard.
Note also that shipping it by pipeline is basically impossible with current tech, which makes distribution infrastructure a real problem.
And when it's all said and done, liquid H2 really isn't very dense, so you either need a big tank or to accept a short range. There is work being done on storing it in a solid matrix (a hydride), which actually gives you better density than a liquid, but I don't think anyone has a product based on that yet.
I'm not saying that hydrogen is without useful application. I can see, for example, using it for large stationary
applications like shifting a few MWh of energy production from night time to peak time.
I'm not even saying that it would be impossible to develop a workable mobile storage system and associated distribution infrastructure. It's just that it would be alot of tech development for very little clear benefit. There are better options: Li-ion batteries look like the current leader of the pack, and that tech will continue to improve regardless of politics or government subsidies
because the electronics industry is driving it. But synthetic hydrocarbons (from pyrolysis) are an option. Even ethanol is probably a better bet than hydrogen, even though it would also require a major infrastructure rebuild.
Hydrogen's advantages are dubious, and the fact that PEM fuel cells (the type that are small, lightweight, and run relatively cool: i.e. potentially good for mobile applications) haven't developed as fast as expected really puts the nail in the H2 coffin for me. Read the opinion
of the European Fuel Cell Forum on the subject. They're dropping PEM work from the forum, because it's not going to pay off any time soon (and there is lots of very real potential with other kinds of fuel cells, for stationary applications).
Bottom line: I think companies like BMW and Ford are pushing hydrogen precisely because
it's sexy, not because it's practical, efficient, environmentally sound, or will ever result in a workable product. It's a PR maneuver. There's a reason I call it hy(pe)drogen.