Korea is a special case due to Chinese involvement. Certainly the Chinese were at our level, but we didn't engage the Chinese in all-out war because of the implied threat of the USSR stepping in. It's difficult to judge the North Koreans separately from the Chinese.
The north Vietnamese were not on our level at all, but we were fighting a conventional war against a guerrilla army. Add in the willingness of the NVA to do things like put machine gun batteries on top of hospitals, and it wasn't so much their training and ability as our inability to fight a guerrilla war. I think that's the biggest challenge with face with Daesh. They have relatively poor weapons, and outside of civilian areas, they're "vastly inferior forces in the dessert". But taking a city from them would prove extremely costly, both in U.S. lives and civilians.
Iraq's army was a decent army, probably better than they are given credit for due to the shortness of that war, but they also played by the rules of conventional warfare. So we took out their airfields and armored convoys and destroyed their missile batteries and they largely surrendered without resorting to guerrilla warfare in the cities.
The last unbridled slugfest involving the U.S. between roughly equivalent forces? WWII. Everything since has been half-measures on one side or the other, or the U.S. trying to fight guerrillas with conventional tactics.
