Numerous problems: parasitic mass, and the fact that the hydrogenation is exothermic and the dehydrogenation is endothermic (i.e. requiring energy from the hydrogen you produce, to run that reverse reaction).
All hydrogen storage systems suffer from either parasitic mass, extreme energy loss at source, high energy loss at destination, energy loss in transit, or low energy density per unit volume, or a combination of these.
The problem is the hydrogen molecule's properties, so we're unlikely to innovate our way out of it.
