The unification of Italy and Germany would challenge what in Europe?
There were many seemingly undesirable consequences of German and Italian unifications in Europe, but the biggest one of all was the Balance of Power.
The Congress of Vienna had established a balance of power between the Continent's biggest powers: Britain, France, Russia, Austria-Hungary and the Ottoman Empire.
German unification interfered with this balance. The germanic tribes that made up the symbolic "Holy Roman Empire" were a significant military power, but were weakened by squabbling amongst themselves. Uniting these tribes introduced a powerful military presence into Europe disrupting the balance established by the Congress of Vienna.
Italian unification also threw a wrench into the balanced structure, as well as being concurrent with several independence movements in the Balkans, weakening Austria-Hungary.
Ultimately European powers formed a new set of alliances and balances that one observer called "a powder keg."