So true - extremely difficult to test kites up against each other, almost impossible...
I have done it a lot, but time consuming like nothing else
First you have to use a lot of time on the kite, so you know it sufficiently well, as all kites need a different way to be flown and handled.
Then you have to fly the 2 you want to compare, neck and neck same day, changing back and fourth and TRYING to get your muscle memory to work reasonably even when changing often.
You have to change so many times, that you rule out differences in wind - as we are talking about the low end or ultimate low end here, so it means the world...
If you get a new kite, it will take about 3 months with a lot of riding, till you can utilize this kite well, and almost half a year till you can max it and fly it blindly.
Many dont know this - they just go out and fly a new kite (which is easy, we can all do that), and get their impression out of this.
It will only give a very rough impression, and for sure not about the low end, as this is so dependant on "feel" which takes a long time to get for a new kite.
All kites can (and should) stall, so talking about a kite that dont backstall does not make sense IMO.
Some are more prone to this than others though, and this can be presented (whether it is good or bad is a different thing though, when you know how wings work...)
If you trim so lots of rear line tension, ALL kites can stall.
Personally I never ever fly a kite that I can not backstall when trimmed open and bar pulled - but this is a personal wish for maxing it, others want the opposite, that they can pull the bar and the kite wont stall
The latter is not possible though in all conditions, as in lighter wind every kite will stall