Green hydrogen is very energy inefficient. You have significant losses in hydrolysis (using electricity to liberate H2 from water), compressing and cooling the gas, and in generating electricity from it.

Anything that is energy inefficient is an expensive energy source, unless the energy that goes into making it is very cheap.

H2 has the advantage of having the greatest amount of energy per kg, and thus is much more attractive than batteries for use in aircraft. It is also relatively clean burning.

The disadvantage, other than cost, are storage technologies (it embrittles metals and requires high pressures and/or very low temperatures), the fact that it is explosive in air, and burns with an invisible flame, and the cost and material (and energy) to create a hydrogen distribution infrastructure.