How to grant only certain roles permissons over certain field?
Field permissions module now has a release candidate version for D8, this is the way to implement Role-based fine-grained control over fields. 

When you enable that module edit the Content type fields you want custom permissions on, you will see a new option Field visibility and permissions where you can choose Custom permissions. 

When you tick that option a table will appear listing Create, Edit and View permissions and all the Roles you have defined on the system. Add the Reviewer role to the ones who can Edit the field (Administrator is granted all by default), and adjust the other permissions as needed. 

A new menu item will appear under Reports > Fields > Report all field permissions /admin/reports/fields/permissions where you can see all the Fields custom configuration in one place.