I don't see why you say this. I wouldn't say they are especially competent, but they seem to have stayed in existence and played off various larger rivals successfully for 70 odd years.
They are as "successful" as cancer is "successful" - it's hard to kill the cancer without killing the patient, but that's not a redeeming quality for cancer. In the same way, it's hard to kill Kim's regime without subjecting 25 million people to extreme dangers, exacerbated by the fact that many of them are already on the brink of starvation, and a lot of them are thoroughly brainwashed. It's not a sign of "success" or regime's rationality, it is the sign that it is hard for us to find a solution for this without hurting a lot of people in process.
A rational actor is a nation that is predictable, and is willing to uphold their word, and agreements. If promises are broken, agreements torn a part I don't think that state is a "rational" actor.
Personally I would trust the USA on its word far more than I would NK. I don't know about you.
No it isn't.
A rational actor is one who acts rationally.
If survival and self-interest is your goal then lying and breaking agreements might be a perfectly rational approach.
It depends entirely what is to be gained.
North Korea acting unpredictably and dishonestly has little inherently to do with the rationality of those actions, and there are strong arguments to be made that those actions are rational from the perspective of the North Korean ruling class.
