Cognito, as with all AWS tools, feels like strange alien technology and it takes some time to understand why it exists and what needs it serves. If they called it the "Amazon User Profile and Sign-In Service" they might have greater adoption.
More examples are always good, especially around using Lambdas to query or extend the Cognito data store.
Amazon Cognito has a bit of a learning curve. You need to learn its concepts and terminology. The documentation does not describe some topics comprehensively.
Some Console screens would benefit from improved search and filtering options.
When another AWS product (e.g., SageMaker) configures Cognito on your behalf, it is not clear what you're getting. For example, the expiration of a temporary password was configured but never communicated.