Baloney. I’d guess she just makes a lot more money off selling Allegany saddles than any of the synthetic brands. If she sells you an Allegany saddle, she can charge for the saddle fitting (10 templates, lots of photos…). A name brand (Wintec, Abetta…) synthetic saddle would yield much lower profits for her.
Is an Allegany saddle nicer? Sure. And the cost is correspondingly higher. Used ones on their web site are all priced at over $1000 with many over $2000. Used.
Are synthetics made with a cheaper tree? Well, sure. That’s a given. You can’t make a $300 saddle with an expensive tree. So, sure, they’re not typically going to be as “sturdy” as an expensive, well-made wooden tree.
“Upper weight limit” is a slippery concept to define and would be a function of many factors, but saying that a synthetic western saddle has an upper weight limit of under 140 lbs. is an untruth.