Could this cause an apocalyptic worldwide famine if applied to plants that form large parts of human food supply (e.g. by terrorists, rogue states, or as part of the "destruction" phase of a mutually assured destruction)?
Probably not much more than current crop seeds do, insofar as large volumes of crops are already highly homogenous GMO breeds. I think this technique will probably just be a better mousetrap for GMO breeding.
It'd be easily detectable-- any phenotypic differences would present themselves in the first generation. Unless the rogue actors were able to add gene drive genetics to a wide swath of a plant's population in a short amount of time, this doesn't seem a tangible threat.