Auth0 solves pretty much every problem I've encountered. Which is admittedly, not a whole lot. It's not that these are super hard problems, it's just that it often seems like a waste of time. Instead of working on authentication and authorization, I'd much rather focus on the actual functionality.

Problems I encountered:

* Normalized user profile using data from various identity providers [1].

* Passwordless login [2].

* Using custom SMS gateway for passwordless login or 2FA [3]

There are not a stable, polished, feature rich alternatives to Auth0 that are affordable. I know that's a lot to ask for, but since you're asking. Auth0 is very nice, but prohibitively expensive. There are also not a lot of maintained and stable alternatives. Pretty much the only one I am aware of is Keycloak.

[1] https://auth0.com/docs/users/normalized/auth0

[2] https://auth0.com/passwordless

[3] https://auth0.com/docs/connections/passwordless/guides/use-s...