"Plagueis was able to kill the comatose Darth Venamis and then bring him back to life several times, before the Bith's organs failed, and Plagueis granted him everlasting death."
Plagueis used it on himself to heal his own injuries. Yet it could not heal venamis's organs or could it? So what is the limit of this powers healing capabilities?
Only information we have on this event is from Darth Plagueis novel. Note that in following passage, Plagueis did resurrect Venamis several times until Venamis's organs could not endure anymore this procedure. Then he tried to heal himself but didn't completely succeed. This implies that repairing damaged organs is perhaps harder then bringing someone back to life. Note also that Plaeguis finally did go too far and Force rebelled against him.
On the same day they had allowed Venamis to die. Then, by manipulating the Bith’s midi-chlorians, which should have been inert and unresponsive, Plagueis had resurrected him. The enormity of the event had stunned Sidious into silence and overwhelmed and addled 11-4D’s processors, but Plagueis had carried on without assistance, again and again allowing Venamis to die and be returned to life, until the Bith’s organs had given out and Plagueis had finally granted him everlasting death. But having gained the power to keep another alive hadn’t been enough for him. And so after Sidious had returned to Coruscant, he had devoted himself to internalizing that ability, by manipulating the midi-chlorians that animated him. For several months he made no progress, but ultimately he began to perceive a measured change. The scars that had grown over his wounds had abruptly begun to soften and fade, and he had begun to breathe more freely than he had in twenty years. He began to sense that not only were his damaged tissues healing, but his entire body was rejuvinating itself. Beneath the transpirator, areas of his skin were smooth and youthful, and he knew that eventually he would cease to age altogether. Drunk on newfound power, then, he had attempted an even more unthinkable act: to bring into being a creation of his own. Not merely the impregnation of some hapless, mindless creature, but the birth of a Forceful being. The ability to dominate death had been a step in the right direction, but it wasn’t equivalent to pure creation. And so he had stretched out—indeed, as if invisible, transubstantiated—to inform every being of his existence, and impact all of them: Muunoid or insectoid, secure or dispossessed, free or enslaved. A warrior waving a banner in triumph on a battlefield. A ghost infiltrating a dream. But ultimately to no end. The Force grew silent, as if in flight from him, and many of the animals in his laboratory succumbed to horrifying diseases.
When the time came for change in leadership of Sith Order, Plagueis could not use this power on himself. Maybe because it was difficult, and maybe he lost his powers because Force resisted his power to create life .
Plagueis gulped for air and lifted an arm toward him. “There’s the rub, you see,” Sidious said in a philosophical tone. “All the ones you experimented on, killed, and brought back to life … They were little more than toys. Now, though, you get to experience it from their side, and look what you discover: in a body that is being denied air, in which even the Force is failing, your own midichlorians can’t accomplish what you’re asking of them.” Hatred stained Sidious’s eyes. “I could save you, of course. Return you from the brink, as you did Venamis. I could retask your body to repair the damage already done to your lungs, your hearts, your aged brain. But I’ll do no such thing. The idea here is not to drag you back at the last moment, but to bring you to death’s door and shove you through to the other side.” Sidious sighed. “A tragedy, really, for one so wise. One who could oversee the lives and deaths of all beings, except himself.
