Turboman
Mab An Gov
On paper that sounds perfectly reasonable, but why does an empty transit van tow a car transporter trailer in a far more stable manner than the same loaded trailer on a SWB landy. This is exactly the thing I have experienced, the transit was lighter too?
Probably because you didn't get the weight distribution quite right when you were towing with the SWB, my guess would be too little noseweight, that can be almost undriveable. Too much handles ok, just heavy on the tow vehicle, and juicy.
Other factors could be tyre type or pressures on the tow vehicle, often people don't pump up the back tyres enough, tyre pressures on the trailer, or uneven side to side loading.
There is no reason a SWB cannot be set up to give a perfectly safe, predictable and pleasant tow. Either of the trailers in the pics are totally relaxed to drive, comfortable with one hand on the wheel, although I don't do this in practice.