West Germany welcomes a 1950s-1960s society - East Germany into a 1990s world. That part of Germany is still a drag on the German economy plus it brought in Merkel. I am not saying it was bad and they shouldn’t have done it. Just pointing it was not easy and it wasn’t cost free.
South Korea will be integrating into its society a functionally a medieval level society, where the average North Korean of military age has the physical (and probably mental!) development of a South Korean 12 year old. This will be a huge huge challenge! That’s why all the SK’ers I talk to aren’t that enthusiastic. The ones that are living here in the US tend to be the more entusiastic, they won’t have to live with & pay for the process. Many SK’ers in South Korea actually prefer the status quo as sad to contemplate as that is!
It won’t be easy at all. But the alternative of war is much worse.