So-called Sage Chakra was created by mixing physical energy, spiritual energy, and Natural Energy in a 1:1:1 ratio.
If Natural Energy made up less than one-third, Sage Chakra could not be used. If it exceeded one-third, it would consume the user, possibly turning them into a frog statue like those on Mount Myōboku. Jugo's mental breakdowns and berserk episodes were also a result of being unable to freely control Natural Energy.
If one was not sufficiently skilled at maintaining this ratio, things could easily go wrong. Cultivating Sage Mode meant taking on that risk.
Zhen had truly tried condensing Sage Chakra inside Jugo's body, but it was far more difficult than he had imagined.
The main reason was that people could absorb a fixed amount of external Natural Energy, whereas Jugo could not. He seemed to have merged with that Natural Energy itself, with it coursing freely through his body.
No wonder Jugo had never been able to control himself freely in the original story. Controlling the Natural Energy in his body was like controlling all the Natural Energy in the natural world. Even if the First Hokage were still alive, even the Nine-Tails' Jinchuriki probably would not possess such an enormous amount of Chakra.
Using Jugo's bodily fluids, Orochimaru had created the Cursed Seal, allowing those marked by it to use Sage Chakra.
If a Cursed Seal user became skilled enough, they could wield that power freely. Yet Jugo, the source of the Cursed Seal, could not do so, and that was precisely why.
Zhen felt that he could also imitate the Cursed Seal from the original story and create a Sealing Jutsu that would allow the human body to produce Sage Chakra on its own.
Like the Double Four Symbols Seal, this Cursed Seal could automatically convert Chakra.