Fission arguably went from the first serious attempt at a reactor (U of Chicago, Dec. 1942, successful self-sustaining reaction) to powering a large, high-performance naval vessel (USS Nautilus, Jan. 1955, in service for ~25 years) in 12 years. Its technological and social problems are very complex, but pretty well-known.

Fusion experts have yet to achieve any self-sustaining reaction - in spite of numerous, well-funded attempts going back ~60 years. Roughly speaking, there seems to be no plausible prospect for fusion being a viable power source for anything outside of a lab. So its technological problems amount to "not actually possible", and its social problems amount to "very cool waste of money".