The description on Wikipedia makes it sound trivial if you have high-pressure, high-temperature plumbing, and a lot of risk tolerance. What is it leaving out?

I think those 3 things are the complexity

Development and discovery of appropriate catalysts and ability to organize the large industrial scale needed to produce meaningful amounts of product.

Can't you just use finely divided magnetite, or am I thinking of the wrong process? That sounds like "you can build it yourself" to me.

Lab demo scale, sure. The research and testing of catalysts for the industrial scale was a significant part of the development effort. Lifetime of the catalyst, temperature and pressure of the reactor, input energy, and output rate were all considerations and the result was much more than just throwing some magnetite powder in a reaction chamber.

There's a great book on the development of the process The Alchemy of Air https://www.amazon.com/Alchemy-Air-Jewish-Scientific-Discove...

Thank you very much!

Well it's better than Mościcki's process :)

What a coincidence, I passed today in front of his house and the chemical plant he was in charge of. The whole area is named after him too.

I did not expect to see his name in HN.