In his writing in the notebook, even Mister White had admitted that his thoughts on the Xuan Yellow Sutra were only theories. He hadn't tested any of these predictions in the real world. To find out how much power the geno art would really have, Han Sen would have to practice it.
Mister White did mention, however, that there were risks associated with practicing the Xuan Yellow Sutra. The geno art could theoretically reverse a person's bloodline, but doing so would be very dangerous. Attempting such a feat would have consequences throughout Han Sen's body. If his body was unable to withstand those effects, the ramifications could be dire.
Han Sen was very tempted to learn it because of the scary powers that the Xuan Yellow Sutra offered, but the risks were severe enough to make him hesitate.