I've only used trailers with central wheels, and loaded them in such a way to balance over that pivot point. Uphill riding was still a lot of work. The central wheel design isn't so much about making the riding easier, as it is about not overloading your rear wheel...that's the wheel that already carrying most of your own weight, and - in most configurations - the bulk (if not all) of the cargo loaded directly on the bike.

The basic problem with trailers is that you can put a lot more weight on the trailer than you ever could on a bike. Also, an extra set of wheels adds more rolling resistance. One wheeled trailers also make balance more complicated, and can introduce side to side torques on the frame that will fatigue the bike over the long haul.