The entry in the [official Star Trek Encyclopaedia][1] states that Borg implants are added to each drone **depending on the needs of the collective**. 

Different eyepieces are seen, presumably because the drones will need different sensing capabilities to do their assigned jobs. It makes little sense to outfit a Medical Drone with the ability to target objects at great distance, for example.

![enter image description here][2]

Tactical Drones, for example seem to have long-lens type eyepieces whereas Medical Drones appear to have a microscoping eyepiece with multiple lenses. Normal 'maintenance-type' drones seem to have the bog-standard holographic eyepieces that we see in I, Borg.

![enter image description here][3]

  [1]: http://en.memory-alpha.org/wiki/Star_Trek_Encyclopedia
  [2]: https://i.sstatic.net/8QO6z.png
  [3]: https://i.sstatic.net/wgSND.jpg