Emet described what the sundering did in the demonstration with Ryne: "Two individuals, identical in appearance, yet reduced in all respects. Strength, intelligence, the soul itself - all is halved." Externally they may have looked the same, but they were 1/14 the person they used to be and devoid of their memories.

Also, the sacrifices were never the real issue. Based on the LL, Venat always intended to sunder the Ancients because she believed (her personal opinion, not objective truth) that they were incapable of change or of defeating Meteion. She needed both Zodiark and for the star to be restored, which is why she didn't intervene in those sacrifices. The third one didn't matter in regards to her plan. I don't believe it's explicitly stated, but given that Elidibus had exited Zodiark in an attempt to mediate and Hydaelyn chose to sunder Zodiark at that time it may have been a calculated move on her part. I certainly wouldn't put it past her to have orchestrated the divide in society to allow for such an opening, especially given that her followers in Anyder seem oblivious to her true intent and the subsequent consequences of it.