Serbs are an opportunistic race, they had fought valiantly against the Mongolians under Ottoman banner, but they have turned against the Sultan as the Sultan said no more pillage in a holy war. They were the ones to assist Ottoman troops to Thrace, but when they felt their homeland was under threat, they never hesitated to stand against the mighty Ottomans. Suleiman got killed in such a struggle. Serbs are unpredictable, you need to have a real tight leash to keep them aligned with your interests!
EDIT:Balkans are prone to more turmoil largely because of the Ottoman policies, and Soviet style unification is no longer on the agenda,