Challengers are limited in their capacity to tow and load pull scrapers by their traction, or lack of it. Their original design function was to avoid soil compaction. They do alright with smaller pans, but these bigger pans (25-35 yds) require a heavier tractor with much more tractive ability. This is especially so in soils that can be slippery (clay or loam, top soil) or in rocky or hard gravel. I'm sure DPete has had all kinds of experience with his in these situations. Even D9's have trouble loading pull-pans in some cases where it's wet or very soft, or in very hard materials. But then, all scrapers do.
I guess that crawler tractors and pull scrapers ink out in some situations, but I'm not sure that double-barrels would not do as well or better. I know I've been involved in estimating situations looking either way, and even if the costs were tilted better towards cats and pans, the long term efficiency and capabilities of the rubber-tired machines penciled out better. Cats and pans are not an off the shelf rental item either. Those firms that use them usually already have some in inventory. Somehow, they make it work.