What is the difference between Unlimited RT and Safe RT in Final Cut Pro?  RT refers to Real-Time or Real-Time Effects.  Real-Time Effects are effects that can be applied to clips in an edited sequence and played back in real time without requiring rendering first.  In Final Cut Pro, you have Real-Time Playback options, which allows you to control the quality of Real-time playback.

Safe RT assures that no frames are dropped during playback, although you will have to render before you can playback.  Unlimited RT will play as many frames as possible, and allows you to play more effects but will also increase the possibility of dropping frames.  Sometimes it's best to use Unlimited RT when using Motion or LiveType with complex effects or composites.

You find the RT options in the RT pop-up window in the upper left corner of the Timeline.  Click on the arrow and it will give you the Safe RT and Unlimited RT options.

Just remember to be sure that you're rendering everything that needs it, set your sequence for Safe RT; that will force you to render everything that needs it based on your processor power, but it still doesn't guarantee that you won't drop frames due to a slow media disk.

What really matters is which is important to you?  Visual playback quality, or the number of effects that can be played back in real-time.