In short, federation system is against national identity of "Republic of Korea"(hereafter abbreviated as ROK). It is because ROK or South Korea has adopted unitary state system since a new nation building in 1948. Regarding national form, unitary state system has never been expressly declared in the successive South Korean constitutions. Notwithstanding the fact, it is regarded as an unwritten or customary constitutional law that ROK is an unitary state. As a result, pursuing unification of South and North Korea by federation is clearly unconstitutional under ROK's Constitution.
Related with this, South Korean president shall have a sincere responsibility to ensure and defend the continuity of a state according to the Article 66 Paragraph 2 of ROK's constitution. So ROK's president and government under his/her control should seek after a peaceful unification policy on the basis of free and democratic order, maintaining a national identity of ROK as a unitary state. If the South accepts and pursues unification in accordance with the North's proposal for unification by federation, it will necessarily bring about renunciation of national sovereignty by reducing ROK's independent state status to local autonomous government status. In other words, South Korea will be equal to North Korea which shall be considered an "anti-state organization" under ROK's constitution. This means denying ROK's national legitimacy and its government's soleness and lawfulness on the Korean Peninsula. After all, pursuing Korean unification by federation formula will be in grave violation of Territory Clause, Article 3 of ROK's constitution and of other constitutional norms as well, including Unification Clause(Article 4), Democratic Order Clause(Article 8) etc.