The most difference is in our cloud service. Previously you got one organizastion within the ZITADEL. In v2 you will get a complete ZITADEL instance with as many organisations as you like. Also the pricing changed a lot. We have a huge free tier with all features enabled.
We also improved how to install and use ZITADEL locally. Our ultimate goal is to get you up and rinning within 5 mins. No matter if in the SaaS or with the opensource repository.
Disclaimer I work at ZITADEL and am one of the co-founders.
We always recommend not building auth by yourself. In the first place it may look simple like its only two input fields and a button (username, password), but to get a really secure solution its a lot more to do. You might need some more authentication methods like passwordless, mfa or identity brokering with google, microsoft, etc.
With ZITADEL we built a solution that combines best of Auth0 (great SaaS solution) and Keycloak (opensource). We believe that a cloud SaaS solution is great to go if its possible to use a cloud solution, but there are always lots of on-prem use cases, thats why we are opensource. https://github.com/zitadel/zitadel
ZITADEL is also focused on B2B usecases, so you can have multi tenancy really easy.