Feet/ankles on humanoids are really hard mechanically. In many ways the kinematic requirements for the ankle are similar to a wrist, but while the wrist of a robot arm is the least-heavily-loaded, the ankle area can be the most heavily loaded. Humans get away with it by having most of the highly-stressed actuators in the lower leg, not the ankle itself, whereas many humanoid robots end up putting more of the actuators in the ankle assembly itself.

In general, I think the best way to think about the differences between human muscles and robot actuators is that human muscles are simultaneously incredible in terms of strength and power density, and also incredibly fragile. Robot actuators are fairly robust, but comparatively poor. Humans are essentially falling apart at all times, but the repair mechanisms in the body do a good enough job that it doesn't matter (although you probably know someone with a "career-disruptive/ending" sports-related injury that shows these mechanisms have limits). Robotics is a long way away from making actuators that can be fixed online in such a process. Even cable stretching in cable-driven mechanisms remains hard to handle effectively, and that's one of the simplest types of mechanism wear.