If it was anyone else, they would use this move in a rather reserve manner, after all, no one would be able to bear losing a part of their soul, unless they have beings just like Chaos, Harmony and Grim protecting them from irreversible damage, also because of the feedback given to him by Sans and the others.
His spiritual world manage to keep stable even after hosting three supreme beings from the spirit clan, he might be the only one capable of doing this, with the feedback of all summoned and contracted beings he currently has, his soul strength outmatched that of a True Saint.
Unfortunately, also because of this, he had to build a powerful foundation for his body to bear with the Saint transformation, according to the records of those that have achieved Saint Realm, it seems that their soul have undergone a baptism.