Think about it, what happens when you sit on a flex tree ? IT flexes out to its wider flex. Whats the difference between a flexed out and a tree that is that wide to begin with ?
ORrrrrrr it is stiffer and you sit on it and it does nothing. So then whats the difference between it and a tree the same as it in its unflexed state ?
I imagine if you put someone on it that just so happens to be the exact perfect weight to allow a small amount of flex without maxing it out you may gain some kinda advantage. Would be pretty insignificant. A flex tree saddle isnt going to be some magic fit wand, Saddle is still going to fit or not fit. No more likely than a traditional tree.
Saying that it does seem that people that have Abetta's do like them. I wouldnt spend a ton on one and if you like it and it fits go for it. But don't buy it think because "flex" is in the name it will fit any horse you put it on.