Well, with the combos I've towed close to rated capacities, I never felt like I wanted to tow more than 3 or 4 hours, or more than a few hundred miles. And certainly not in mountains. I've towed in mountains when I had a combo with reserve capacity so I know what the difference feels like. Like I said, on relatively flat terrain, towing close to the capacities isn't bad. But even then those combos would downshift and sometime struggle even on relatively low hills especially in the wind. It was a limitation I would live with.
There is a marked difference when the combo is "right", that is has some reserve capacity. With my current combo my truck doesn't even break a sweat.