Hello Mark,
The Mr Shifta I have seen is a well made product, and it is capable of moving caravans, BUT, the fact that it fits to the hitch or A frame of the caravan endows it with some important limitations.
There are basically two types of caravan movers, those that fit to the 'A 'frame, at the hitch or jockey wheel, and those that drive the main wheels of the caravan or axle movers.
All movers need to be able to grip the ground, and most of us instinctively know that to get better grip you need to push the driven wheel harder to the ground. Not enough down force and the wheel will spin, or skid.
The problem for all 'A' frame movers is the down force is limited to the nose weight of the caravan + the weight of the mover. This in practice is not much different to the weight of an adult, so if you find that you cant get enough grip to push or pull the caravan, then an 'A' frame mover is likely to experience the same lack of grip under the same conditions.
On the flat that wont be a problem, but on a slope it means the caravan may run away and the mover will not be able to stop it. The ability to grip is also dependant on the direction the caravan is facing on the slope. Don't forget there are many conditions where the quality of the grip you get will be reduced, for example, wet leaves, gravel and of course ice and snow.
By comparison the axle movers, have roughly half the weight of the caravan on each wheel, which radically improves the grip, and most importantly the stopping ability on slopes. The larger diameter wheels also make it easier to move over uneven terrain and loose material like gravel.
Another factor for some people is the availability of the mover. The 'A' frame movers almost always have to be detached, and stored or carried each time you want tow the caravan, where-as the axle movers are always there and easily made available at home or on site.
I know of places where the Mr Shifta would be very effective to move caravans and other trailers around a flat storage compound and workshop, but it would struggle on the entrance ramp from the road to the compound.
Horses for courses as they say.