I rode a smaller WB gelding in one for a while, he was a bit over 15hh with a rather stocky build and the 16" "off the rack" fit seemed very comfortable for him- the smaller saddle and horse combo worked well. Absolutely looooved the saddle, would have gladly left the horse and taken his tack!
When my wide-but-not-stupid-wide DWB was in his mutton-withered stage long ago (9 years or so), I tried one. It was almost wide enough. His idea of wide is at least 5" between the "dots" or top front corners of the panels. But it rocked back and forth as I posted.
I think that it had a problem common in many french saddles. The waist of the saddle is narrow and the panels are relatively thin so there's a very small footprint on the horse's back. For this horse, wide enough in front sometimes meant not enough thickness to the panels farther back. That plus the banana-shaped tree sealed the deal. The horse has saved me some coin: No saddle of this design will work on him.