The Soul-eating physique is a curse.
At the same time, it is also a blessing.
The essence of Soul Assimilation is to devour the Spirits of the Five Elements and become their master. However, the Spirits of the Five Elements are omnipresent, and they possess consciousness themselves. Thus, they also aspire to be the master of the body.
Therefore, every Spirit Assimilator, after ingesting a spirit, will undergo a lengthy period of being besieged by the Five Elements. This period is quite painful and protracted for them, as the Spirits of the Five Elements will use various methods to shatter the Spirit Assimilator's inner strength, eventually occupying that perfect body.
In this prolonged confrontation, if the Spirit Assimilator wins, they will possess the physique that has the Five Spirits Subordination, and from then on everything will be smooth sailing.
If the Spirit Assimilator loses, then it's a total defeat.
"I won."