Quite simply put, some of the embedded dialogs like the Advanced Find, Merge Records, Assign window are not by default visible on the Unified Client
Like when you multi-select records, you can’t see the typical Merge, Edit buttons on the ribbon.
Here’s how you enable them
In your Web Application’s system settings, you have an option under general where you can enable these buttons in the Unified Interface
Now, you can retain those buttons in the Unified Interface as well
Have a great time exploring Unified Interface!