That's the point of the in-line wheels. They work on sidehills, unlike side-by-side wheels.
They balance very well if the load is balanced. If you watch the video you can also see how the handles can be changed to work in different terrain.
With heaver loads more people help, but the handles are designed so 2-3 people don't get in each other's way. But because of the hand-brakes, one person can do pretty well with even a fairly heavy load. I took two field-dressed pronghorns (about 150-160 pounds) out of some badlands last fall by myself. With two guys in decent shape 300 pounds isn't bad at all.
The in-line wheels also allow them to go over fairly sizeable rocks or logs pretty easily. The guy with the mule deer on his Neet Kart demonstrates how that's done.
