Thanks for all of the informative posts! The reason why I was considering ANA was
1) The UA + ANA flight is $400 cheaper than the all UA version. ANA has some really good deals for NRT-BKK (34000Y).

2) UA only has one flight per day for NRT-BKK and the times aren't to my liking. ANA has 4 flights per day with much better times.

UA
6:30PM NRT - 11:45PM BKK
6:35AM BKK - 2:15PM NRT

I'm going probably go with UA+ANA and take my chances. Is there a special security line for Premier when I arrive at NRT?

If UA had kept their 7pm NRT-SFO flights (that were canceled earlier this year), then my life would be less stressful.