Is Start Screen Unlimited safe for using?

There is no system files patching or some other illegal tricks. Start Screen Unlimited is absolutely safe for installing and using.

Does it work on tablet devices?

Yes. It will work on any tablet with Windows 8/8.1.
Surface RT is not supported.

Unlimited is not displayed on the Start Screen

Check that your version is 3.0 or higher.
You should access to the Internet and check for Windows updates. It should fix the problem.
Refer to the Microsoft article: http://support.microsoft.com/kb/931125

Unlimited does not work with multiple monitors

Check that your version is 3.0 or higher. The issue has been fixed.