Funding aside, the Manhattan & Apollo projects were (by comparison) simple, with clear objectives, and a compelling rationalisation.
Manhattan needed to build a vialbe, air-transportable, nuclear bomb. It arrived at two independent designs, both effective to task, and succeeded in assimilating materials for at least several more weapons (on the order of 6 IIRC), over the course of WWII. The rationalisation was both defeating, and not losing military supremacy over, an enemy. (Germany was also working on a nuclear weapon.)
Apollo needed to land a man on the Moon and return him, alive, to Earth. Rationales varied, but one compelling argument was as a proof-of-capability of US technological capacity as regards missile design, reliability, control, targeting, with the additional constraints of solving life-support systems challenges capable of keeping meat-puppets alive for a week in space. Drag-racing and Lunar golf were bonus stretch targets.
Each project was relatively independent of other complex technological systems, and wasn't concerned with a tightly-coupled, long-term, widespread integration. Manhattan's gadgets needed to fit in Bock's Car's bomb bay. Apollo's designs, once the fundamental mission was completed, were largely abandoned. Several technologies survived, but the project as a whole did not.
Both projects saw numerous exceedingly close approaches with disaster. Manhattan's tail-tickling proved unwise, the US are still cleaning up Hanford (and as yet have no final management plan), and there were several near-catastrophic handling mishaps at Oak Ridge. (Though in total the project was remarkably smooth.)
Apollo's record was, to my knowledge, far worse. An entire crew were lost in ground testing, one very nearly in space (13), and missions 8, 11, 12, and probably others, saw one or more potentially fatal / mission-loss incidents and events. Margins for safety were extraordinarily slim.
(Subsequent history with the Space Shuttle and two total vehicle losses shows that risks persisted decades later.)
The Global Energy Sustainability Project is far more complex.
It is building not a simple tool or one-shot mission, but a total system. That system needs to be tighly integrated, as an integral component, of the worldwide technical economic system. It needs to function from now until doomsday, or at least until some replacement regime is established. It challenges what's arguably been the most lucrative and critical as well as geopolitically salient sector of the global economy, which is to say it upsets a vast set of economic, financial, political, military, and national interests. Pains are localised, benefits are diffuse.
Success depends on global coordination and cooperation, or barring that, sacrifice and effectiveness on local scales which counters the actions of any defectors.
This is almost certainly the Biggest of the Big Problems.