The Kurds did well defending their territory but we cannot forget two things about them

1) They sacrificed the Yazidi (spelling) people. The Kurdish Regional Government maintains that the Yazidi province was their responsibility and under their control, yet when ISIS came they retreated. One reason they retreated was because of the religion issue. According to Islamic people in the region, Yazidis are considered dirty people who worship 'shatan' (satan). The second is the Yazidis, though of Kurdish ancestry and language, wanted autonomy (and independence) from both Kurdistan and Iraq. They were recorded as saying 'We are not Kurds', so, they were punished via non-intervention.

2) The Kurds did little to nothing to help non-Kurds in a military style. They outright refused to help retake non-Kurdish cities, and did not help at all on any offenses including the Mosul offensive. There are a lot of media reports about how much they were doing but, I can attest, these were lies. During the war, it was very important for Kurdish people to appeal to the outside world and thus had an interest in appearing helpful, moral, stable, etc. Although they are by and far much better people than Iraqi Arabs, they are not morally sound nor helpful to any causes but their own. Middle Eastern standard I suppose.