Towing without a WDH is safer. It enables the rig to retain stability at higher speed.
For on-road stability, a conventional caravan needs to be nose-heavy by 8-10% of its laden weight. When hitched to its tow vehicle that (typically 200-350 kg) pushes down on the rear of that vehicle. As with pushing down on the handles of a wheelbarrow, that levers up the front of the tow vehicle , thereby reducing the weight on the tow vehicle’s front tyres.
Where that tow ball weight is comfortably within the laden tow vehicle’s payload, that front weight reduction is unlikely to cause any but a slight decrease in stability.
If that tow ball mass results in the tow vehicle running at its maximum legal weight, a so-called Weight Distributing Hitch (WDH) may then be added to resolve the weight problem. In doing so, however, it fixes that weight problem - but introduces unwanted effects.
How a WDH works
A WDH is, in effect, a springy beam that, by levering up the rear of the tow vehicle, restores weight (down-force) from the tow vehicle’s rear tyres to its front tyres.
The WDH, however, can only counteract the caravan’s tow ball down-force. Side forces when the caravan yaws (sways) are still imposed unchanged on the tow vehicle’s rear tyres that now (due to that WDH reducing the weight on them) are less able to counteract those side forces.
The overall effect is to reduce the rig’s ‘cornering power’. In extreme circumstances, these side forces may cause that tow vehicle to oversteer. If that happens, a jack-knife is inevitable.
That WDH reduces the rig’s stable top speed. That does not imply rig inevitably misbehaves at speed – but is more likely to if ‘hit’ by a strong enough side force.
Limiting a WDH’s unwanted effects
You can limit a WDH's downside by correcting only 50% or so of the weight transferred. Check this by (before coupling up the caravan) measuring the distance between the top of the tow vehicle’s front wheel and the wheel arch. Then, with the WDH not engaged, couple-up the laden caravan. Then adjust the WDH such that the the distance between the top of the tow vehicle’s front wheel and the wheel arch is halved.
Do not attempt to level the rig.