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!