Engines.
".... even the N1's engine has turned out to be a star on the Atlas V."
Difficult not to be. As it's not only a closed cycle unit, but an oxygen rich one to boot, its efficiency is off the scale compared to anything NASA have produced.
They built it the old way: Take plans, build engine, test engine, BANG, fix bit that went bang, test engine, BANG, repeat until working engine achieved, redraw plans from result. NASA's "ticks in boxes" style involves going back to redesign / planning whenever anything (including BANG) goes wrong. They reckon it would have taken them at least 30 years to go through the number of design iterations the Russians did in two.
But NASA and their contractors have a process and a focus on safety, which makes it all ok....