Can 'incomplete expression' be a synonym to 'understatement'?

No. An understatement is litotes, a statement which minimizes what it is talking about. "The United States has some debt problems" is an understatement because the United States has a huge debt problem. "Robert Wadlow was a bit taller than the average" is an understatement because he was the tallest man ever.

An incomplete expression is one which is not finished. "Oh, for--! Give me that!" includes an incomplete expression.