Not just a little controller, a counterbalancing mechanism with gyroscopes, etc. Versus a motorised trolley.
if the 4-wheeled version must also have the same level of acceleration and speed of picking up, won't it also need a counterbalancing mechanism? It just won't need the "balancing algorithm", but will need a way to balance the arm swinging out.
Also, the counterbalancing mechanism is written with software, so once perfected, it's "free" to deploy. The weight could be the actual batteries, so no added extra weight!
Do you see cars with huge counterweights?
And no, it is not free to deploy by any means. Any moving, huge, swinging parts will cause expensive problems. Remember your hard disks.
Of course it depends on the weight of what you are moving, but it doesn't seem like the 2 wheeled robots in the video can actually move "heavy" weights.
I would not call 15 Kg "heavy". I can carry that myself on a single arm...