Yeah, it's a shitty army, but, as you said, it can still fire artillery shells. :(

As for your example of East Germany, the unification of Germany was voluntary. I don't think the average North Korean would be too happy to be conquered by, say, Japan. (And I don't think the people in East Germany would have been happy to have found themselves becoming part of France.) And insurgents don't have to be supporters of the previous regime; they could simply be out for themselves, or follow some other cause opposed to that of the occupying forces.

Mostly, though, invading North Korea just isn't in anyone's interest. There simply isn't enough wealth to steal to make it worth the billions of dollars it would cost to send an army to invade and occupy it.