When it comes to perfect accuracy of ancestral/ethnicity mixtures, yes, you shouldn't rely on DNA tests to be the final say in what countries your ancestors came from.
However, getting your DNA tested can lead to you identifying others who may share ancestors with you, and by extension, discovering which ancestors you thought you had really are related to you, and which aren't. This can be used to more accurately discover what your ethnicity is than a DNA test.
DNA tests are not going to give you every answer you're hoping for without you putting in some work. But they do open the door for exploration that a large number of people (such as adoptees) did not previously have access to.