The obstacles faced by the NERVA project were immense, and the iterations achieved spectacular improvements in record time. That project that lasted for less than one decade and was done on a shoestring budget is not quite the equal of the Manhattan project, but it's not much below it. It achieved the highest density of power generation by maybe a factor of 100 compared to any other reactor in history.
All that stuff was pretty much lost. NASA is trying to revive this, but it will not have legs. We simply don't have the same pool of talent to recruit from. In the 60's there were lots of scientists and engineers that had first hand experience building nukes and nuclear reactors. Now this country has not built a reactor in 3 decades, with the exception of Vogtle.