For whatever it is worth, I've carefully watched the videos on the Northwoods site, and I think they are different because they are specifically designed for off pavement towing, unlike my trailer. The frame is much beefier, for one thing. So is the axle and the suspension.
So since I have an ordinary ultra-light trailer (made by Cruiser), I have upgraded the axle, installed beefy leaf springs, and heavy duty shock absorbers, all with the goal of off pavement towing.
In order not to bust the lightweight frame or the standard-duty coupler, I go about 5 to 10 mph on very rough roads (up to 20 on good gravel roads). I have been doing this for the last 15 years, with no problems. Obviously, most of our towing is on pavement, with only the last few miles of each trip on forest roads.
But if I were going to do a LOT of off pavement towing (more than a few miles at a time), I would get a trailer built for that purpose, like one of the Northwoods models.