On another thread recently, I had noted that releases of hydrogen would not persist for long in the open air due to the fact that this gas is lighter than air and would disperse accordingly. Delving deeper into the subject, I found that hydrogen burns under an exceptionally wide range of concentrations, 4% to75%, vs. 5.3% to 15% for methane. Hydrogen flames also propagate more rapidly in enclosed volumes of space, hence there is greater risk of explosion compared to other flammable gases, and will ignite with a very weak spark- all of this making special safety precautions necessary.
Hydrogen is odorless and burns with a very pale, almost invisible flame(this last consideration is of course immaterial in the event of explosion) and adding odorants and/or chemicals which would make the flame more evident will ruin the gas for use in fuel cells and further dilute the already low energy per unit volume of the gas.
Hydrogen embrittles many metals and is incompatible with existing natural gas pipelines, is less than a third the energy density of gasoline per unit volume even when in liquid form(at or below H2 boiling point of -422.8 *F/-252.8*C) and requires a great deal of electricity to produce via electrolysis of water using current technology.
Given all this, the prospect of the widely touted "Hydrogen Economy" coming into being seems remote.