Solution:
There is no known drawback with using WP’s login system, it’s WP after all, you can allow people to log in/out from the frontend using Sidebar Login
Yet, the only drawback I see with this is that it will be so obvious that you are using WordPress and you won’t be able to customize the look and feel of the backend as if you were using Drupal (where you can set backend themes).
Other than that, you will probably want to:
- Modify roles: User Role Editor
- Remove some admin pages and menus : Admin Menu Editor
- Remove unused profile fields : Remove Fields