I had the same intuition, but then I remembered the toy I'm playing with - the spectacularly unstable 4-rotor copter that could not even take off without an onboard computer actively keeping it level all the freaking time. And it's rock solid almost no matter what.
As technology changes, our intuitions must follow.
2-wheeler is not better or more stable than 4-wheeler and requires more maintenance.
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...
Wheel issues (typical):
1) flat tire (if the wheel is not "solid" only)
2) axle bearing
Controller+swinging counterweight:
1) a gazillion electronic components, connectors and contacts, bearings as well, the actuator for the swinging, etc., etc.
What happens if the environment is a bit humid?
For the wheel, a little rust, for the complex electronic system?
And BTW, we can save 25% by using only three wheels ...