Looks like they've taken end carts a step further. Possible
downsides are narrow wheels if you cart across soft surfaces and low ground clearance on rough terrain. At one point in the video, the yak was dragging over an obstacle.
End carts in general are quick and easy to put on the yak. Being on the end, you are carrying most of the weight.
There are less expensive end carts if you're looking for an end cart to use on firm, smoothish ground....Paddleboy's original large runs $100