In the 54 years I have been driving, I have towed many different types and sizes of trailers, using many different tow vehicles. I have never used a sway control gimmick. In fact, for many of those years, they had not been invented!
I used a WD hitch for one and only one tow vehicle, towing a 19 foot TT. That Jeep Wagoneer HAD to have a WD to keep the back bumper off the ground!
IMO, a properly built, properly loaded trailer will NOT sway. If it does, there is a problem. Find the problem, and FIX IT, and THEN if a sway control will help you sleep better, get one and use it. BUT, a sway control should NEVER be used to solve an existing sway problem. It will MASK the problem, but it will NOT CURE it!
I had one homemade flatbed trailer that had a sway problem. No matter how it was loaded, it would sway, usually when going down hill. It was NOT built properly! Nothing could be done to stop it from swaying. Sway controls were not available at the time. I sold it with the clear understanding that at the very least the axles had to be relocated before it was safe to use.