It depends a bit on exactly what your use case is (i.e., which features are even relevant to you) and specifically what the alternate vendor would be. I really do try hard not to oversell what we offer.

I'll say this: if you're looking for an open source solution built to make B2B auth easy, we're the only option of the three. For Keycloak, for example, it's been an unresolved feature gap for many years that it can't support SCIM provisioning [0]; an unrelated characteristic -- not necessarily a flaw -- of Keycloak's is that it's quite agnostic to your use case and consequently requires some effort to implement.

On the other hand, we don't yet offer a huge amount of customization. We've instead -- for now -- prioritized building a relatively opinionated and therefore straightforward product. Similarly, if you're looking for some long-tail features, a longstanding vendor like Auth0 will very likely be able to help you.

Probably a few other considerations might be relevant that I'm neglecting. Send me an email if you're interested in chatting more (contact in HN bio) -- more than happy to help explore some of your options, even if that means we're not right for you.

[0] https://stackoverflow.com/questions/58566587/does-keycloak-s...

Thanks for evaluating FusionAuth! (I work there.)

Would love to learn more about the strengths and weaknesses of each of these options in light of your use case.

My contact info is in my profile.

Have a look at Authkit.