For a start, it's worth noting that nationalists in the Kingdom of Serbia were very keenly aware of the Sardinia-Piedmont analogy. The journal of the Black Hand, the group that assassinated Franz Ferdinand, was called Pijemont. The idea of uniting the South Slavs was key to Serbian political ideology, and Serbia viewed much of the Balkans as 'unredeemed Serbdom'.
I think the key problems, though, are language, religion and identity. For all the various troubles in the Italian peninsula, the Italians spoke very similar languages (the place where we draw a line between a 'dialect' and a 'language' is fairly arbitrary), almost all of them were Roman Catholic and the Roman Empire was strongly remembered as a great and praiseworthy nation. These shared factors served Italian unification well. None of them were present in Yugoslavia-to-be; there were religious differences (which can be used to rally separate identities around), there were considerably greater linguistic differences, and, perhaps most importantly, someone in the Balkans living under the rule of a foreign power (e.g. the Ottoman Empire) mostly didn't think of themselves as Yugoslavs, but as Croats, Slovenes et cetera; there was no strong shared memory of a united Yugoslav state that everyone could be proud of, despite Serbia's largely unsuccessful attempts to revive the idea of Stepan Dusan's empire.
The fact that Serbia committed various atrocities against other people in Yugoslavia-to-be is, I think, a result of the fact that the non-Serbs in Yugoslavia-to-be did not enthusiastically embrace the Serbian 'liberation', because they did not view themselves as of one people with the Serbs, for the reasons above. Had (e.g.) the Albanians considered themselves a Yugoslav people and enthusiastically welcomed the Serbian presence, I imagine they would have been treated much more kindly; as it is, the Serbians, awakening to nationalism, felt under threat in their quest to bring about 'greater Serbia' and 'unite Serbdom', and responded, accordingly, with great harshness.