Slipping? Rolling would be more like it!
I thought a treeless would be the answer to fitting my wide-shouldered short-bodied little paint pony. I got a good one, too - ordered a Trekker from Germany. Along with the recommended Mattes pad. For what I spent I could absolutely have gotten a Duett or similar treed saddle made for wide horses. And I could've ridden in it without ending up under the horse.