It would replace non renewable fossil fuels and nuclear plants (not environmentally friendly)
It produces electricity and pure drinkable water. (2 important features of life)
Hydrogen is also non-toxic, making it a rare fuel source. Nuclear, coal and petrol are all either toxic or found in hazardous environments.
It leaves no harmful biproducts, if we used it in NASA shuttles it would also provide clean water for the astronauts.
It is highly flammable - this means there are many risks
It is a light element, which rises into the air, bonding with other elements which is harder to isolate.
It is very expensive, as it takes a lot of work to isolate. Akso it is very time consuming to make.
It isn't easy to replace the current infrastructure, All cars, planes etc would have to be replaced at a high cost.
Some people would argue "why should we replace our current fuel sources which work so well."