How did you make sure that no one would complain that "Bob was an idiot who kept breaking the build", or "Alice never does her code reviews and blocks everyone"?
Good approach: Specify that complaints be non-specific to a person on the team.
Very often, it is the case that a specific person /is/ the problem.
If you identify a problem, the person responsible will (usually) know you're talking about them, and may even appreciate that you're not calling them out in front of the team.
If necessary, you can always reach out to them one-on-one afterwards.