hydrogen contains a lot of energy per kg but its density is extremely low, so you need a huuuge and heavy high pressure container to hold it. That mostly negates all its advantages, at least for the aeronautics industry and the contemporary space industry.

It would be a different story for spaceflight if one had the capability to fuel up a hydrogen rocket in orbit. In that case one could (mostly) take full advantage of all the benefits of hydrogen over other propellants.

It depends on how it's generated. There are a few methods, each with its benefits and drawbacks. These are referred to as the "colors" of hydrogen generation.

• Green Hydrogen is produced by electrolysis. Obviously for this to be eco-friendly it needs to be powered by renewable sources - currently, as renewables are a growing sector, some argue that those sources would be better used for directly powering the grid, or other uses like carbon capture, etc. I'm not here to say which is actually the best use of renewables, just what some of the arguments are.

• Blue Hydrogen and Grey (or brown or black) Hydrogen are produced from fossil fuel sources like natural gas (grey) or coal (brown/black). This process produces CO2 as a byproduct and can either be captured and sequestered (blue) or not (grey, brown, black).

• Other methods include "turquoise" hydrogen which uses pyrolysis, and produces solid carbon which can easily be sequestered, and "pink" hydrogen which uses a nuclear power source to perform electrolysis.

EDIT: It may also be considered a bad fuel source because it needs to be stored at high pressure, which presents engineering challenges and can be dangerous (obviously, hydrogen is highly combustible. The Hindenburg used hydrogen to stay afloat and look how that worked out). I think there are also concerns about its energy density vs other fuel sources but I don't know as much about that.

It's not a matter of "hydrogen bad" so much as it's a matter of "hydrogen not gonna do what Hype Science Magazine says it will do, at least not yet."

Promising faster cheaper easier transit is today's incarnation of the boy who cried wolf. Replacing what we have with something better is what vast many claim and scarce few deliver.

It's less of a "source" of fuel and more of a storage method. If you're using electrolysis to make it, you're taking electrical energy and using it to convert water into a store of energy. That has inefficiencies at each step (generation, transportation, storage, and finally in its use.)

Currently the cheapest way to obtain hydrogen is by extracting it from petroleum products, the process releasing carbon dioxide and other byproducts.
Ultimately you'll get less useful energy out of it than if you just used the fossil fuels outright.

Hydrogen is already somewhat expensive compared to other fuel sources, so getting most buisesses to obtain it in environmentally friendly ways would be a challenge.

There's also the issue of storage. Gaseous hydrogen requires large pressurized storage tanks, and ends up being less energy dense in terms of volume than other fossil fuels. Liquid hydrogen of course can be much more energy dense, but requires a lot of energy to cool it to the point where it becomes a liquid state, and further cooling to keep it there.

Although liquid hydrogen has its use as a specialized high energy density fuel, at the current moment, I don't see hydrogen as an economicaly viable fuel.

Currently the cheapest way to obtain hydrogen is by extracting it from petroleum products, the process releasing carbon dioxide and other byproducts.
Ultimately you'll get less useful energy out of it than if you just used the fossil fuels outright.

Hydrogen is already somewhat expensive compared to other fuel sources, so getting most buisesses to obtain it in environmentally friendly ways would be a challenge.

There's also the issue of storage. Gaseous hydrogen requires large pressurized storage tanks, and ends up being less energy dense in terms of volume than other fossil fuels. Liquid hydrogen of course can be much more energy dense, but requires a lot of energy to cool it to the point where it becomes a liquid state, and further cooling to keep it there.

Although liquid hydrogen has its use as a specialized high energy density fuel, at the current moment, I don't see hydrogen as an economicaly viable fuel.

It’s not that it’s a bad fuel source but it seems to be very hyped and currently not very economical nor is it as safe as other fuels (highly flammable etc). In order to accommodate it, other prototype aircraft have had to remove space for passengers because of the size of the fuel cell required, for instance.

I would be the first to say that I am open to persuasion and am by no means as well educated on it as I could be so always happy to be directed to reliable sources of info.

Generating hydrogen with electrolysis (using electricity to decompose water into Hydrogen and Oxygen) is energy intensive, and most is created with fresh water, and this could create another resource scarcity issue. However, seawater electrolysis is being researched and it looks like a breakthough was achieved at the end of last year.

If the electricity used for this does not need to be stored for later use (unlike an EV), then it is more efficient to use it immediately (solar and wind farms often need to store the energy they generate during off peak hours for peak usage hours).

If the electricity used for this process is generated with fossil fuels, then it is more efficient to just use fossil fuel combustion directly for energy instead of first generating electrical energy, then storing that energy in hydrogen through electrolysis, and then re-releasing it with hydrogen combustion. Or, if hydrogen gas is really needed for some reason then it can be synthesized with processes that use the fossil fuels directly.

TLDR is energy is lost from the inefficiency of energy transformation which is needed to generate hydrogen.

Third, using Hydrogen cools down various propulsion systems due to pressure difference when distributed to the combustion engine.