North Korea is a strong counry alright. But am talking about the economic strain it might put on the South Koreans if united. There will be stigma toward north koreans, the poorly developed north vs the south, idealogies etc will definitely be challenges. Not to mention, healthcare and education which is totally different in the north. Will take a lot of effort to bridge these gaps.
There won't be a merger between the two countries. They hate eachother too much. A change of leaders won't get rid of that animosity.
Not to mention you'll have the issue of that ridiculously large minefield between them if they do merger