I for one can tell you that other international carriers RIGIDLY enforce their carryon rules. When I had to ZED on Qantas when I was stuck in NRT, my carryon was sized fine, but was "overweight". It was the only bag I had with me, so I was forced to check it.

Thank goodness I had an Australian visa, because I had to enter the country to claim my bag, then proceed to the UA counter so I could go home