Self-balancers are a milestone in your robot building life ... He chose steppers over cheaper DC motors because this delivers precision and avoids issues when the battery voltage drops.

[Aaed] decided to try and build a quasi-direct drive actuator, similar to those used in MIT’s agile mini Cheetah robot. It consists of a powerful brushless DC motor driving a 9:1 planetary gear ...

Understanding the Structure of Brushless DC Motors ...

Designing robots requires precision and flexibility ... affect how precisely the motor is controlled. Brushed DC motors can be used in some low power cases such as hand and finger control.