The engineering problem for wide spread use is hydrogen
embrittlement.



What, from burning it in your engine or from manufacture?

From containing it.

Essentially the same problem as ethenol; existing plumbing won't
handle hydrogen, so if you want to distribute the stuff in anything
other than liquid hydrogen trucks, you need all new pipes designed
to carry hydrogen.

Hydrogen atoms are so small, they will wedge themselves even into the matrix
of solid metal materials. This reduces the ability of the metal atoms to
slide alongside each other, hence the material will break rather than bend
if force is applied - the material becomes brittle.

regards,
Friedrich