Well on the whole the different ethnicities of the KUK regiment were largely dependent on the location of the regiments. For example the Jager regiments recruited primarily from the western half of the monarchy so there was a disproportionate amount of Germans and Italians represented in those units. The cavalry drew large amounts from the Hungarian (Hussars), Polish/Ruthenes (Uhlans) and Czechs (Dragoons). In contrast, the mountain dwelling Slovaks and Slovenes rarely featured in the cavalry regiments. The artillery took largely from the German and Slovenian populations of the monarchy as these ethnic groups were typically better educated and had the technological expertise, and so they rarely recruited from the less educated Serbs and Romanians in the monarchy, who generally made up the rank-and-file infantry. Now this covers how the ethnicities were distributed within the KUK regiments, so lets go over the combat effectiveness of the various ethnicities. Now there is the general misconception that the Hapsburg soldier was an unwilling conscripted soldier fighting for a state he felt no loyalty to, but this misconception is largely the result of postwar entente propaganda. On the whole, the Hapsburg soldiery fought loyally for their Emperor. However there were several stereotypes that were perceived within the Hapsburg Armed forces, whether or not they turned out to be true. To the Hapsburg high command, the Italian soldier was a reluctant warrior, Austrian mountain peasants were fine soldiers and Tyroleans were excellent marksmen, but were also slow moving and stubborn. The Hungarians were perceived as brave but unruly and violence prone; Czechs were reliable but unimaginative; Poles were heroic but wild; Ruthenes were believed to be meek and dumb; Romanians were backwards and primitive; and the Serbs, Croats, and Bosniaks were the finest soldiers in the monarchy but they needed to be lead well. To elaborate on the Bosniaks, these were Muslim serbs recruited from Bosnia-Herzegovina which had been annexed by Austria in 1908. They were given distinctive uniforms and Fezzes to wear and quickly earned a reputation for their savage tenacity on the Italian front during the First World War.

Sources: Beyond Nationalism: A Social History of the Hapsburg Officer Corps by Istvan Deak

The White War: Life and Death on the Italian Front by Mark Thompson