The UN mediation and general work in Palestine was objectively a failure.

Korea... it preserved South Korea's dictator in power, which allowed for a modern democratic and prosperous South Korea to happen. Back then it was little more than protecting the US-backed dictator against the Soviets-backed one. Both were pretty terrible and murderous.

In regard to Korea -- it was also about the principle of maintaining recognized borders, and their involiability. The UN was also instrumental in bringing the conflict to an end (along with Stalin's death and the general state of exhaustion on both sides -- but nonetheless, it was instrumental). And yes, they were both awful dictatorships at the time (and the South would continue to be, for decades to come) -- but's also not like there isn't a considerable difference between the two societies, now, generations later.

Palestine - many failures, but there've also been many important resolutions that have kept the conflict (at least somewhat) framed in terms of the RBWO and the rights of the region's indigenous inhabitants.

We also have the Geneva Conventions, etc.

So in sum - yes, many failures, but on balance I see the glass as more half-full than half-empty, on this issue.