Being that this is an international forum perhaps we need to put in the definition of understatement.

understated: avoiding obvious emphasis or embellishment

An understatement would not be false; instead it would be true, but not providing sufficient description of the magnitude of something.

It's like a surgeon getting ready to cut into you without benefit of anesthetic saying, "This might hurt a little." The natural response to understatement would be, "Well, DUH!" or "No, $**t!"