In recognition of Null's justified criticism I will say that while my answer provides a good explanation of the physical degradation advanced dark-side force users experience, as well as an explanation for a limited number of cases where Sith have been forced to become at least partly robotic, it is perhaps only one component of the answer OP is seeking since it does not directly address the cases of the Sith he specifically cited.
The dark side energies that the Sith channel are actually physically harmful to living bodies - the biological form of a Sith Lord tends to wither over time, especially as they grow more powerful. For reference see this article on Wookiepedia, specifically the section titled "Corruption of the Dark Side" where it is stated:
"The dark side, much like a strong, addictive narcotic, affected not only the mind of the user but the appearance as well. As one immersed oneself deeper within the dark side of the Force, its malevolent power took a toll on the body...because flesh and bone lacked the endurance to channel the immense power of the dark side indefinitely."
This is in direct contrast to the light side energies channeled by the Jedi, which are specifically tuned for the preservation and augmentation of life.
As an example consider what is perhaps the the ultimate expression of Sideous' force abilities in the films: lightning. The energy that he conjures is incredibly frightening and powerful and has absolutely no practical application aside from destruction. This energy is literally coursing through Sideous' body at every moment, and the "current" has weathered his physically-human form terribly over time.
Compare this example of dark side energy to the light side of the force and you will see that there is obviously no real equivalent. The dangerous energies that flow through a Sith Lord's body are not present in a Jedi, and it is because of this that light-side force practitioners do not suffer the physical ailments and disfigurements so common to the Sith, including their need for mechanical replacements for failing organic systems. From the same article linked to above:
In time, the corruption could go beyond mere cosmetic details and directly impair physical abilities. King Ommin of Onderon was a proficient Sith sorcerer for most of his life, and the dark side held him under its decaying influence until he eventually became incapable of movement and needed support from a cyborg exoskeleton in order to survive. Another extreme case of dark side corruption was Darth Zash, whose extensive study of the dark side led to immense physical degradation and eventually a fatal terminal condition. The worst case of this transformation would be Darth Nihilus, who was completely consumed by the dark side both mentally and physically. Eventually, he became an aberration of the dark side that existed only to consume life.