This is something the United States did not want to see. A major goal of US foreign policy from the 1990s to the present was to ensure that all Russian allies (actual or potential) would be wooed or forced into the Western orbit. The drive was to do this while Russia was weak. This has been largely accomplished through NATO and the EU. Obviously the most contested places like Ukraine and Georgia are well known. The Post-Yugoslav Balkan Wars of the 1990s were part of this same strategy with Serbia, (historically one of Russia's closest allies) being the most resistant.
Serbia has been broken and brought to its knees but the process of fully integrating the new puppet regime has been difficult. The society is not stable and there are a lot of very difficult and tangled negotiations that have to be worked out in order to bring stability between Serbia and its neighbours Bosnia, Croatia and the state wrested from Serbia... Kosovo. Croatia is now in the EU and NATO while Bosnia is aspiring to join certainly NATO and possibly the EU.
The Brexit opens a window of opportunity for Vladimir Putin in the Balkans. There are forces within Serbia and Macedonia that would rather ally themselves with Moscow than Brussels and he certainly will do all he can to exploit this angle. The situation in Macedonia is complicated and is becoming ever more volatile. As Putin is being pushed from almost every side, some leverage in Southern Europe will help him diplomatically and geostrategically. And yet, his hands will be somewhat tied. The resistance to Russian incursion in the form of military exercises or trade deals will be fierce.
If the Brexit had been avoided then it's likely that in the next five years or so these nations could have been brought into the EU-NATO orbit and forever ripped away from their historical ally in Moscow.
These things can't be openly discussed but this is one (of many) aspects of the Brexit that are proving problematic for the grand architects of geopolitical strategy.