There is so much custom code to essentially fling data between services. I imagine only a small percent of the code and dev time is spent on business logic. These Rube Goldberg machines are my least favourite part of AWS, especially when dealing with at-least-once delivery.
What's wrong with it?
If the appeal is about free usage under 40k monthly active users, you probably don't need an external complex managed auth solution in the first place.
Visit the amplify-js and amplify-cli github repos and search for cognito issues.
Try using the cognito console, it is a litter box of warning messages and exceptions. Try connecting Cognito and Pinpoint if you are in Europe, 2 years and it still doesn't work, but no indication.
See its cloudformation configuration and how many field changes cause a Replace. Yes, replace Cognito and goodbye users.
Until recently user names were case sensitive, and they didn't have basic account enumeration protections.
Its UX is exactly how DynamoDB can do damage to a product.
I understand the wish to keep everything in AWS.
Especially after I read many times thaz Auth0 is pretty expensive. Is this true?
I would have preferred to keep everything in AWS just to make it easier to use one set of credentials (IAM) to manage permissions.
That's when I figured that I'm not gaining anything by using Cognito.