I met a few Europeans (and others) touring on flat bar bikes. Most of their bikes looked like older style hybrids to me. Some of them had the bikes set up to sit bolt upright. I would have found it uncomfortable and in the open country where we met them would have hated the wind resistance, but they all seemed happy with their choices. I personally would have set mine up less upright than most (all?) of them.
I remember in particular a Dutch guy on the Southern Tier, Martin I think, who was very tall and sat bolt upright in the very open country of the south west. The winds were pretty strong at times and I was thankful to be able to get a little aero on my bike. He didn't seem to mind though. I guess he is used to dealing with the wind in his home country. It looked like torture to me. He probably thought the same of my setup I guess.
His very upright posture would have killed my back with the road shock going straight up my spine, so I'd have at least set up the bars so I was a little less upright even with a flat bar setup. I have ridden many miles on mountain bikes with flat bars set up with a good bit of tuck in my position and found that for me it worked well. I do miss the hands on hoods position of drop bars though and have even done mountain bike single track riding at times with drop bars. I never found that bar ends were a great, let alone perfect substitute for the hands on brake hoods position.