Comment by michaelt

It's a simple matter of the number of motors you have. [1]

Assume every motor has a 1% failure rate per year.

A boring wheeled roomba has 3 motors. That's a 2.9% failure rate per year, and 8.6% failures over 3 years.

Assume a humanoid robot has 43 motors. That gives you a 35% failure rate per year, and 73% over 3 years. That ain't good.

And not only is the humanoid robot less reliable, it's also 14.3x the price - because it's got 14.3x as many motors in it.

[1] And bearings and encoders and gearboxes and control boards and stuff... but they're largely proportional to the number of motors.