You'll be looking at combinations of old reviews, open bench, pre applied TIM on aio, no cut out on top or at front of case for rad, aio running in balanced not custom, cpu not pushing the cooler, GPU not running at the same time, fans upgraded on air and not rad like you say and so on. Air is cheaper and easier to live with. It won't perform as well as the latest 280 like the XT from corsair if it's well setup. Ram clearance is also an issue on air and if it's avoidable, case clearance can then become an issue from the fan height.
Fitting an aio to the front of the case with a solid panel in front of it in silent mode with it's pre applied TIM is completely different to fitting one in the top of the case or front of the case with a fully meshed panel with custom fan speed and your own TIM. You may as well be using a different cooler. These variations don't exist with air. Lots of cases don't have large meshed panels for rads. In turn, these aios can't push/pull the air properly.
Everything you described can be applied to air cooling. Not all cases are the same, some are too restrictive etc. Loads of different mount, TIM, fan options for Air as well.... I stand by what I said, the best air is better than the best AIO in terms of performance (by single digit %, so not much). Custom loops eclipse both of them. Not much more to it than this.