I'm actually using it on about 40 units (maybe 42) in my company.
Never heard about different hardware between them.
I basically can't see significant differences between them and my own iPad air, using a quite heavy software for aeronautical charting and navigation.
What I meant by "different hardware" between them has to do with manufacturing tolerances and elements of the hardware that fail/falter over the life of the device. Apple allows a range of results during testing that they consider "passed". It is possible that the impact of those near the boundaries of acceptability is felt more with subsequent versions of iOS.
My sense is (and it could be wrong) that there seems to be an increase in the disparity of user experiences the older a particular generation of device gets and the number of updates are applied to that generation.
Apple tests the latest hardware with the latest version of iOS and they adjust both ends to attain the desired results. But once that generation of device is out in the field, there is less inclination and less resources to thoroughly test and adjust iOS updates for previous generations of hardware.
Then there's the application mix which is a variable that can't be ignored. I don't deny that there is a wide range of experiences given the same hardware and iOS version... but I can't dismiss it all as the rantings of people with an agenda.
That's different than using a variety of components of comparable specs (which is something that companies like Dell do... the running joke a few years ago was that you can tell which component contractor was the low bidder that week based on the week the device was built---components changed that frequently).
Apple may do that (screens perhaps?) but I'm not aware that they do anything like that with other components.