Two things WD set does.
1. It equalises the load front and back. Not enough allows the back to lower and raise the front depending on the hitch overhang. And WD set up usually add to this overhang, requiring even more balance. To much reduces the amount of rear axle loading and loads the front and trailer axles and even the trailer and TV frames.
The second effect of WD is to center the trailer with the TV thus limiting sway. Friction bars do not allow that to happen and might effect the method of getting out of a sway situation. Not enough load will increase sway and to much will cause front steer pressure also cause a sort of sway which to me is very criticle.
Furthermore towing a trailer without WD can be very succesefull if the driver learns the basic principles.
More hitch load means better control and less sway. As the rear axle becomes the front steering axle of the trailer. Short overhang allows more hitch load while limiting the steering axle lift. And less swing when steering left or right.
I never use WD while towing any trailer but do make sure its well loaded. At least 15 to 20 % tung weight. No matter the vehicle.
I always feel I have to steer the TV as if I am driving on ice. If sway happens the only way to control it is by very small motion of the steering wheel. Just like race car drivers controlling a loose car.
I do have lots experience driving on ice here in the north and going 60 is never a problem. So controlling a trailer without WD is similar with plenty of traction
I was delivering a sail boat for our son this summer and I realized that the hitch load was to light for the 3000 lbs combine weight of boat and trailer. It was not setup right by the previous owner. At least not my way for sure. But with the 7500 lbs truck I proceeded anyway. Boat trailers usually do not have WDs due to traction problems for wet ramps.
On the way home it rained hard and in one curved underpass I hit into a large flow of water at least 6 inch deep and the truck went into a skid with the boat and trailer following.
I did the control motions mentioned above and in about 6 left and right small steering motions I was heading straight in my lane. It happens fast but on ice I am use to steer clear of trouble and apply it to sway recovery.
BTW in 40 years its the one and only time of getting in an alarming situation and I owe my success to proper pin weight for the load. And I towed overloaded many times.
Barbara and Laurent, Hartland Big Country 3500RL. 39 ft long and 15500 GVW.
2005 Ford F250 SD, XL F250 4x4, Long Box, 6.0L Diesel, 6 Speed Stick, Hypertech Max Energy for Fuel mileage of 21 MPusG empty, 12.6 MPusG pulling the BC. ScangaugeII for display..