As others have mentioned here, it is difficult to keep contained, and because it's not very dense at room temperature/low pressures. So the example FTA given is

As others have mentioned here, it is difficult to keep contained, and because it's not very dense at room temperature/low pressures. So the example FTA given is cryogenic cooling. Because of the extreme cold, this can lead to new failure modes for materials. The other issue is that hydrogen when it burns has a very pale blue flame that can be difficult to impossible to see during daylight. It also apparently has a low radiant heat, which makes it difficult to feel from a distance. This makes firefighting and safety a major concern. The first you might know of a hydrogen fire, is that you're suddenly on fire yourself.