[panarky]

I was also genuinely interested in your simple question, "what makes the functioning of all the engines so difficult?"

You received a response with many words but I don't think it contains a real answer, other than "the Soviets couldn't do it".

[nick222226]

Because of combustion instability modes, cascading failures, and finally the simple difficulty of high reliability operation of a piece of machinery that is so highly stressed.

There is a very interesting book about it, called Combustion Instabilities in Liquid Rocket Engines: Testing and Development Practices in Russia.