Engines -- LM certainly underestimated the technical risk associated with designing a powered lift aircraft, especially the risks associated with designing a low weight but rugged airframe with lots of big holes in it. They also underestimated the risks involved in stuffing an LO aircraft with a huge number of mission critical systems. That said, I would offer the thought that designers have to take risks to achieve the improvements in performance demanded by customers.
I believe therein lies the issue with many new, technically challenging, military programs. The big issue is the government’s ability to properly evaluate the technical proposals presented by the contractors.
For a short time I was one of the competing programs and had a chance to see the some of the interaction between the government technical folks and the contractor’s engineers. One of the issues was the relationship between the Air Forces technology development arm and the proposal evaluation. For example, at the time of the JSF selection process, the engineers from Wright-Patterson were heavily into developing hydro-electric actuators. Our design engineers thought that the maturity of the design for those actuators were too high of a risk, however, come proposal time, some of the same engineers from the Air Force technology development side were part of the proposal evaluation. They had a vested interest in seeing hydro-electric actuators on this aircraft and took issue with our risk evaluation.
As with many programs, the seeds of future problems can be traced to the technical proposal evaluations.