Let me give you some non-exclusive positions:
- Players will use the prompts in the book to create character with exhaustive backgrounds and personalities that the DM will be forced to accommodate.
- The player will fill out this without DM input and thus may run contradictory to the DMs world and tone.
- The player will decide aspects of their character before the game starts rather than discovering them though play and/or dice rolls.
- The player will become inflexible and demand what is written be accommodated rather than adapt to the DM, events, or world.
- The player will do this in the vacuum of not being with other players, potentially designing a character that does not fit with the other players.
I've heard every one of these at some point here, and this book is gonna be example A in the Entitled Player Arguments from here on out, regardless of how innocuous it actually is.
For the record, I think these are a great idea and if it wasn't cost prohibitive, I'd buy one for each of my players. But I already know that unless this book has "check with your DM" written on each page, people here are gonna vilify it.