Yes, bending and breaking possible.
You have a long steel bar extending rigidly from the back of your truck to the trailer. The bar is a spring so will bend and return to straight without taking a set. It is a spring. Most WD itches are rated for 10,000 lbs, some as low as 5,000 lbs. The difference is the flexibility of the spring (one is stiffer). As you get into the spring rate (bending the spring) you get more spring tension/resistance/ it takes more and more force to bend the spring (straight WD bar). At some point the spring and the trailer frame will be under so much tension that one of them has to give. Either the spring will bend more or the trailer frame will bend.
It is not going up an incline, but rather stated differently, the transition from flat to an up angle, that puts the most tension on the spring, and thus the trailer frame.
It would be interesting to see someone invent a release mechanism that pops the spring out of tension safely before that point of maximum tension is reached and the frame is in danger.
The truck should not be in danger, but the hitch that holds the spring could also be a point of failure if under too much tension.
When driving slow off road the advantages of a WD hitch are almost non-existent except for taking weight off the rear. The advantages are for safe highway driving (or around town) where the trailer is more behaved.
Take the spring bars off when you get into serious off-road features. Again, a regular, non-washed-out, gravel road should be safe.
