'Green hydrogen' isn't a miracle. It is simply a matter of converting electricity from one form to another. Or in the case of the Scottish buses, converting electricity to combustible fuel. The problem is that at every step of conversion, one is losing efficiency.
With 'green' hydrogen, electricity is first being created from some kinetic or photovoltaic source, with energy loss. Next, that electricity is being used to split water, separating it into hydrogen and oxygen, again with energy loss.
The one achievement at this point is that you have now unlinked hydrogen with carbon-based fuels. Instead of generating hydrogen by reforming methane, you have removed methane from the equation. The problem though is that the methane method is much cheaper than the 'green' method. The only possibility for success here is if the 'green' method becomes cheaper than methane.
Now that we have hydrogen, what can we do with it? Well we can't store it. Not very well, anyway. No container can hold it for very long. So, it must be used within days of generation. There are two things that can be done with it. Either use it as a combustible fuel in an internal combustion engine, or use it to generate electricity through a fuel cell. The second option is more efficient than the first. But one has now lost two steps of efficiency to get back to the starting point - electricity.
Green hydrogen works great in a college science lab. But when it comes to consumers putting down real dollars to power their vehicles, homes, etc., it fails miserably. The consumer is always going to demand the most utility for the last dollar spent. And gasoline/diesel delivers.
