Energy recovery was already doable with traditional actuators through BEMF generation, so the only thing this has going for it is static torque.

The only way i'm seeing this doing things at a fraction of the cost is by enabling the use of a smaller motor and repeatedly winding each spring before doing a task requiring more effort. At that point, why not just use a linear motor and even more clutches to do that winding instead of having to move the entire arm?