Iraqi soldiers left 2400 humvees behind when they fled Mosul despite having a large numerical advantage over the attacking ISIS forces. The performance of the soldiers of the Assad regime has been even worse.
The Kurdish pershmega, despite having fewer military assets and a smaller population to draw from, has repeatedly driven ISIS back. The Kurds are also far closer to American values. The YPG's units include women, Christians (including American Keith Bloomfield, KIA), and Jews such as Israeli Gill Rosenberg. When the Yazidi religious group was persecuted by ISIS, the YPG took it upon themselves to save them.
Instead of supplying the YPG, we supply the Baghdad govt. in the name of "stability." The YPG is far more likely to use our aid well, and far less likely to give it to Iran.