When the resistance of the load, like a very steep hill, is so much the hubmotor slows to less than 10 mph for geared motors, then the motor still pulls the wattage, but the stalled motor will turn much of that power into heat.
If the conditions are fully loaded, that is wide open throttle up a steep hill....
At 25 mph, the motor is running at about it's max efficiency, so say it's pulling 900w, only about 100w is turning into heat.
At 15 mph, the motor is still running pretty efficient, but perhaps as much as 200w is turning into heat.
But below 10 mph, that motor could easily be turning 500w into heat. Ever turn on a 500w halogen work light? Then try to touch it about 5 min later? Cooked your hand right? A 500w heater inside your motor will cook it very quickly. It just can't shed that heat fast enough.
However, if the path or road is less steep, and you can ride slow with 1/3 throttle, that's ok. Even if turning half the watts into heat, now it's only 200w, and the motor can radiate that much heat away. It's when turning half of 1000w into heat that the motor can't cool itself fast enough to keep up with it.
But as a general rule, if the hill is very steep at all, you really need to keep moving faster. Find trails where that is possible, or do those trails only going down. Find the easier way up, or don't ride up them more than about 10- 15 min without stopping to let the motor cool for a long time. 45 min of steep hill will cook off a motor, if you can't keep it turning 10-15 mph.
It's a definite problem, trails here out west were built for mule packing, and nothing wrecks a trail like horse traffic. None of the forest service hiking trails I know of in New Mexico are rideable, even if it's allowed. You have to seek out more recent trails built for bikes, or roads built for 18 wheeler log trucks. Mine roads, skid roads for logging, and 99% of trails will suck for both e bike or motorcycle.
But the mid drives that are coming, they can handle some pretty amazing trails. If you can stay on the bike riding that stuff. If you have the strength and balance for it. The vids I've seen, they don't show old hippies with a pot belly riding that rough trail. They show guys who spend the winter extreme skiing the alps, and the summer extreme mountain biking the alps. Like I was, about 30 years ago.
