The major fear surrounding DPRK's missile and nuclear programs are not primarily associated with the advancements of their technology to start with. It's solely because of the irrationality of the person in charge. Like you have pointed out, Russia and China are more advanced than DPRK yet they have not been perceived as threat to humanity simply because their leaders are considered as sane, at lease to the best of general knowledge. The whole world has to be on the alert when a maniac is holding the codes to a nuclear armoury.