In what sense did the Constitution have fatal flaws with respect to states' rights?
Although the Constitution has obviously survived to the present day, we can argue that it had a fatal flaw with regard to states’ rights because it allowed the issue of slavery to lead to a terribly destructive civil war.
The Constitution did make the federal government quite strong. However, it still left the issue of slavery up to the various states. In addition, it did not make absolutely clear whether the states had the right to secede and/or to nullify federal laws. These questions all helped lead to the Civil War. Since the Constitution left these issues open, it can be said to have had a fatal flaw that could easily have led to the destruction of the country.