Adjustable servomechanism assemblies and associated systems and methods are disclosed herein. An unmanned aircraft system in accordance with one embodiment of the disclosure includes a movable mechanism and a servomechanism assembly operably coupled to the movable mechanism. The system also includes an interface assembly operably coupled to an output shaft of the servo and the movable mechanism. The interface assembly includes an adapter portion carried by the output shaft and an output arm releasably engaged with the adapter portion. The adapter portion includes a first aperture having a non-round surface mated with a non-round surface of the output shaft, and a generally smooth, non-splined, engagement surface. The output arm includes a second aperture sized to receive at least a portion of the outer surface of the adapter portion. The second aperture includes generally smooth inner surface in contact with and rotatable through 360 degrees relative to the engagement surface of the adapter portion.