For all day comfort, postural variety is key. Muscles, tendons, ligaments, etc. rebel when they remain contracted ("loaded") for extended periods: they become sore. They will get stronger, but there are limits.

Holding handlebars recruits muscles in your fingers, hands, forearms, upper arms, shoulders, neck, upper back, lower back and more. The more ways your setup encourages postural variety, the less likely it is you will overwork muscles.

I would get very sore on flat bars, even if equipped with bar ends. Both of my touring bikes have drop bars and aero bars. The aero bar position is good for resting the hands, and also makes it easier to climb hills and ride into headwinds.