Balance, for the most part, woudl be a design issue, not a day-to-day thing, IMO.
If you are taking the time to BUILD a trailer like this, then have your rig weighed(without driver) on each axle, as you would load it. A little math and a bathroom scale or two can do the rest.
My design thughts would be that there are 4 pockets for the tires, so there is no chance of the thing being in the wrong place.
I can see with quads and snowmobiles how it'd be hard to get the balance right. But if you have no variability in where you place the tires, then its going to be the same every time, unless you change your rig.
Again, its a specialized setup for someone willing to do it right, not for the average guy.