There have certainly been cost effective hydrogen powered long distance drones. One potential benefit is that you can run fuel cells, which are silent and quite

There have certainly been cost effective hydrogen powered long distance drones. One potential benefit is that you can run fuel cells, which are silent and quite a bit more efficient than a two-stroke driving a generator. OTOH, getting similar energy density is challenging, as others have said. Although you can come quite close with liquid hydrogen, if you can live with a fuel that's not storable for long times at small quantities in the field.