GetComponent() returns the outline text effect instead of the shadow text effect!

Well, that is a common issue when inheritance is used. The Outline class is actually derived from the Shadow class. So an Outline actually is also a Shadow. Therefore GetComponent<Shadow>() will face two instances on the same object. As far as I remember Unity now searches through the components in the same order they appear in the inspector. So a "hacky" solution is to swap the order of the two components in the inspector. That should fix the issue, though not in a nice way.

The best and most robust solution is: Do not use GetComponent but assign the reference through the inspector. If that's not possible for some reason to make the code more robust you would need to use GetComponents<Shadow>() (note the plural), iterate through the components and filter out the right components by doing some type checks.

 Shadow shadow;
 Outline outline;
 foreach(var c in GetComponents<Shadow>())
 {
     Outline o = c as Outline;
     if (o == null) // component is not an Outline
         shadow = c;
     else // component is an Outline
         outline = o;
 }

That doesn't really look very nice but will work no matter what order the two components have.

is this U need?

 public GameObject Outline;

     Outline.GetComponent<Outline>().effectDistance = new Vector2(3, 3);
     Outline.GetComponent<Outline>().effectColor = Color.black;