Hello Peter,
I think that Stephen has summed up the practical limitations of the hitch drive rather well. It also applies to virtually all "A" frame mounted movers.
If all you need is to move the caravan on a flat hard standing, then they are fine, but as soon as you need one to negotiate inclines, gravel or grass, then their primary limitation is the lack of grip.
JTQ has provide a clue as to their ability, but rather than look at what they can do, consider what they can't do. As with most incidents, it's the ability to stop rather than to go. So if you personally would have any difficulty in stopping a caravan from moving, an A frame mover will have the same difficulty.
For many people the benefit of a mover of any kind is not just to help store the caravan, but to be able to position it on site. So it needs to be available. The hitch drive and most other A frame movers have to attached each time you want to use it. That implies the need to be able to lift it in and out of the caravan/towing vehicle at each end of the journey.
By comparison the axle mounted movers are always there ready to be used. Plus they have far superior traction giving better manoeuvrability on most surfaces and most importantly they will stop a caravan on a slope.
By all means go for a Hitch drive provided you are fully conversant with its limitations.