There are a few API endpoints in V2 Admin API that have been enhanced or modified in a way that is not backward compatible, but they were necessary changes to move forward with our roadmap.
In most cases, when developing for Server API admin calls, you'll end up using both V1 and V2 endpoints.
If you want to call GET /users, /insights, /subscriptions or /workflows/all, then you should use V2 Admin endpoints to stay current with the latest codebase.
Even the corresponding V1 Admin endpoints for those calls will continue to work, but if you're writing new development, please stick to the above mentioned V2 Admin calls.
For all other calls, you can use the V1 Admin endpoints.
I hope that helps. If you have any further questions, don't hesitate to post.