Storing green energy in an explosively volatile form.
That's what makes it exciting!
Almost all of the hydrogen is stored in the form of hydride, so the instantaneously available volume of hydrogen is very limited, insufficient for a large explosion. The hydride needs to pull some heat from the environment to release the hydrogen so it is rate-limited. Also, any released hydrogen rises fast in air due to its low density, as opposed to propane which is heavier than air and tends to pool near the ground, looking for an ignition source.