The open-source nature of AuthKit is pretty different. You can build your own complete custom UI with the React components. Or build your own components from scratch and still use the WorkOS backend.
Outside of that, it's pretty much a drop-in replacement for Auth0. We also have more features, like native SCIM provisioning and a streaming events API to keep your app's database in sync.
Also what if you have an existing email-based account system which works fine - can you use AuthKit to add additional sign in methods like social without replacing your existing system?