I think a key point about the failure of Yugoslavia and indeed the entire situation until Kosovo split was that the Serbs had hubris.
Instead of quietly assimilating the other peoples, they had to push.
I think this is understandable looking at 19th century Serbian history and its constant political strife throughout the 1800s. They even pushed the Montenegrins away.
If you contrast this with what the Parisian did in assimilating Gascony, Normandy, Brittany, Occitan, you can see that they (the Parisian) were far more successful
A few thoughts. In the Balkans the differences were much more religious and cultural rather than ethnic and linguistic. The spoken language was mutually comprehensible it was just written either in the western or cyrillic alphabet. But those like the Slovenes and Croats who were Catholic also looked to the West for their influences and allies while the Orthodox Serbs looked to Russia. Also, the centuries of brutal muslim domination left a ton of bitterness in its wake - not surprisingly.
The French were actually pretty intolerant and oppressive toward minority groups like Bretons, Corsicans, Basques, Alsatian Germans, etc banning their languages from anything official, passing restrictions on its publication or later broadcast over the airwaves etc. The French (unlike the Serbs) were a lot more numerous though. That made it easier for them to impose their will.