• Geases were urges that made characters do what the author wanted without the author having to concoct reasons why the characters were doing what they were doing.
• personalities of dead people inserted into the characters were a means of providing exposition. Each character had different implanted personalities so they would have to tell each other stuff and that would tell the reader what 343i wanted the reader to know.

Geases lead to lazy writing as the author doesn't need to worry about arranging circumstances to force the characters into a line of action nor does the author have to worry about questions like: would this person actually do this? Is it in their nature? Why would this person do this?

The downfall is that we can't anticipate a character's actions, how the story might unfold or come to understand the character. These characters are not people, they are automatons, puppets pulled by strings.

Implanted personalities don't have to work things out so the author doesn't have to write all that working stuff out uh... stuff. He can just pad out a list of exposition that 343i gave him.

The downfall is similar for geases. None of the major characters is a single person. They are flesh robots containing an unknown number of personalities.