On all of the single wheeled cargo trailers I have seen, the "steering" pivot is behind the rear wheel. Nearly all of the reviews I have read mention some (greater or lesser) problems with fishtailing at high speed with loads.
All the trail-a-bikes have the "steering" pivot ahead of the rear axle. I have searched, and have found no mention of fishtailing problems with these. Given how protective (overly so IMHO) many parents seem to be of their children, I would think this would be mentioned if it happens.
I have done stability analysis comparing bumper pull to fifth-wheel automotive trailers. The critical factor that makes the fifth-wheel so much more stable is that the steering pivot point is at or ahead of the rear axle. The distance between the rear wheel contact patch and the hitch provides a lever that allows the trailer to steer the tug. If it is behind, then high speed stability is compromised.
When you compare the load ratings of the SW cargo trailers, to a trail-a-bike, it seems that this hitch location has a significant advantage.