I work for FusionAuth and we have some comparisons on our website, but I haven't built an application with each of these.

I'd say that dimensions that matter vary so widely for each person and use case, it'd be hard to do a great comparison.

Dimensions that matter:

   * open source or not
   * standalone application or library/framework you integrate
   * self hosted or SaaS
   * authentication, authorization, user management or all three?
   * standards implementation
   * integrations with other auth tech (LDAP)
   * which OAuth grants they support
   * documentation and developer experience

And that doesn't get into specific features that you might need. An example: if you want to modify a user object in the middle of a login flow, Auth0 has rules, we have Lambdas, Keycloak has plugins. How are you going to know what features you need without building out at least a sample app?

Oh, and pricing! Lots of the smaller operations (us included) have transparent pricing, but Okta/Auth0 don't.

I wrote out a list of 13 different use cases for FusionAuth ( https://github.com/fusionauth/fusionauth-issues/issues/1002 ) and I am still discovering new ways this coiuld be used. I'm sure that is the case with all these competitors.

It's the old elephant story: https://www.peacecorps.gov/educators/resources/story-blind-m...