I have flown from Atlanta or Chicago or Houston to Tokyo or Seoul or Hong Kong on my way to Việt Nam many times. That leg can last 13 hours up to 16 hours. I fly Economy. On Asian or Delta airlines it can be relatively pleasant outside of the need to stand up for periods of time. On United it is quite miserable. An unbelievable number of people seem to prefer miserable because United carries more passengers than any other airline.
Agree United is one of the worst airlines out there, I had to fly on them frequently from Hong Kong.
I would get an occasional reprieve from the warden and fly Cathay, it was amazing the difference.