There are so many parts of someone's life in their memory that changing everything in their memory completely will fracture their minds.
The best way to keep the mind from collapsing is to be selective about what they change. But this means that they will miss some memories and those missed memories can contain important information that should have been erased.
This is why, most of the time, it is only with luck that they are able to overcome the second obstacle. They called this requirement of luck as the third solution needed to overcome the obstacles.
Even the demigod telepath that they had as a slave couldn't offer any useful help. After all, she too had largely fulfilled the advancement requirement and became a demigod telepath due to luck.
