Backing up a trailer with two separate axles is very nearly impossible, at least without a tremendous amount of practice; it's the same basic physics in play as backing up a car on a tow dolly. You have two pivots in play that have to be kept lined up.
Since access to a great many campsites require one to back in a trailer, this is a very significant drawback.
I suspect there could be some rather troubling shimmy or wander problems at highway speeds, but presumably proper design and damping and such could keep them under control.
