The biggest problem as Brian states here is price.
Auth0 has amazing features however it feels like Heroku pricing which doesn't scale with you especially if you are a B2C which may want to have lots of free signups.
Firebase's Authentication is really easy to integrate
Amazon Cognito is super dirt cheap. It the biggest hassle implementing. For ExamPro we use Amazon Conginito because of course but we did attempt all options.
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