Except the moment you have orbital dominance over a planet then no amount of ground or atmosphere based weaponry matters whatsoever. All they would have to do is drop some mass from orbit and they can cause destruction far beyond even the most powerful nuclear weapons.
What you forget is that weapons are just an application of the sciences. Superior science = superior weaponry, even if the aliens haven’t bothered to “spec into” weapons research.
Agreed, until prosthetics can achieve full parity of both function and sensation then they are only good as replacements for parts that are already missing. No sane person is swapping their hand for one that lacks a sense of touch just as good or better than what they have already, even if it’s mechanically superior. In such a scenario that mechanical superiority is desired they would opt for an augmentation over a prosthetic.