Hello Angela,
The hitch mounted movers are only really safe and effective on flat smooth ground. If the surface is poor like gravel or greasy, they will loose grip. On the flat that is not much a of a safety problem, but if you are dealing with even just a modest slope, if the mover looses grip, the caravan is out of control.
I have used both systems, and on flat factory yard the hitch movers are fine, but for real caravanning, both at home and away, the main axle movers offer far more flexibility and security I terms of grip and control.
There have been some users of hitch mounted varieties that have reported incidents where the mover has lost control, and damage or injury has resulted. In most cases they go on to report that they have sold the hitch mover and moved u to the main axle variety - that is why so many are up for sale at what appears to be great savings.