> a counterbalancing mechanism

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!

A car accelerates way faster.

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.

A car does not have an arm that picks up heavy objects. The ones that do (crane trucks) need outriggers and jacks to stabilise the machine and stop any motion before they can extend their arm.

Fork lifts (no outriggers), optionally with a crane arm/jib, are normally used to move goods inside warehouses.

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.

These robots go only up to 15 Kg according to their designers.

I would not call 15 Kg "heavy". I can carry that myself on a single arm...

What do you mean a counterbalancing mechanism? Multiple gyroscopes? It's just a cheap IMU (includes gyrometer) and an additional control output in their controller.