Reproduction Steps:
1. Open Project
2. Build and deploy to iOS device
3. Press UI button
4. Notice that error is thrown
5. Press GUI Button
6. Notice that error is thrown and app crashes
Note: Both button call the same method:

public void CrashNow ()
{
string foo = null;
Debug.Log (foo.ToString ());
}

Reproduced on Unity: 5.3.6f1, 5.3.7p1, 5.4.3f1, 5.5.0f1, 5.6.0a3
Reproduced on device: iPhone 5 iOS 9.35

RESOLUTION: By design. UI system catches any exceptions thrown from code attached to UnityEvents. The documentation has been updated to reflect that.