I started our team on Amazon Cognito because I was a Solutions Architect at AWS and found it really easy to follow the tutorials and get a basic app up and running with it.
When our team started working with it, they very quickly became frustrated because of the poor documentation. After 4 days of trying to get all the basic passwordless auth working, our lead engineer made the decision to abandon it and try Auth0... and managed to get everything implemented in 4 hours.
The consensus was that Cognito just isn't mature enough or well-documented, and that the implementation does not cater for real world use cases the way that it should. I believe Amplify has made some of this simpler, but I would still recommend Auth0 as it's been bulletproof for us, and is a sensible price.