Redefinitions aside, fully capable AI is right up there with commercially viable fusion power, cost effective quantum completing, and fully capable self-driving cars, as a technology that is quickly advancing yet always a decade or two away.
Space flight?
A man on the moon, or the SpaceX rockets that land and can rapidly relaunch, both feel like hard problems that have been solved, although it’s not the next hard step of intergalactic space travel.