Yes, this is a limitation in our war on ISIL:
The Pentagon has been relying primarily on Kurdish forces to isolate Raqqa, the Syrian city at the heart of the extremist group’s self-proclaimed caliphate. But U.S. officials acknowledge that only Sunni Arab forces should ultimately liberate the city from Islamic State, because Raqqa’s mainly Sunni Arab population could regard Kurdish troops as occupiers.
The Kurds can be relied to fight along their borders. They won't take Raqqa. They won't take Mosul in Iraq. They won't take on Assad in western Syria.
Because the Kurds are too few to want to die for Sunni or Shia Arabs by being their shock troops.
Once we reach the limits of what the Kurds will do in Syria and Iraq, just how do we plan to proceed? Especially regarding Assad?
My sketch of a plan ("Win. Build. Win.") accounted for that in theory. Our actual plan doesn't seem to have even an "in theory" aspect regarding this problem.
Or do we think it is fine to effectively be Assad's ally by taking on our common enemy ISIL, which will free Assad to take on other rebels we find an acceptable alternative to Assad?
And of course, the Kurdish problem reflects the fact that the war on terror is really a lot of separate wars on jihadi Islamist impulses taking place across the Moslem world in an Islamic civil war:
The War on Terror that was declared after September 11, 2001 soon evolved into a Moslem civil war between those (mainly Islamic terrorists) who want a worldwide religious dictatorship run by themselves, versus those representing the majority of Moslems who are getting tired of being threatened and murdered by Moslem religious fanatics. The majority of Moslems are not against the idea of a global Islamic dictatorship but that plan has never worked and most simply want a better life in a nation that reflects their own local culture as well as “universal Islam.”
The reality is that the War on Terror consists of many individual wars in which local power struggles, often centuries old, have become more violent because Western forces, seeking to eliminate base areas for Islamic terrorists attacking the West disabled local dictatorships that had long kept the local Islamic terrorists under control. But since the 1990s that traditional control has been breaking down anyway and, as has happened so often in the past, the West sent its own forces to deal with the matter.
So we have many limits in our mosaic of struggles to keep jihadi terrorists from inflicting collateral damage on the West as they fight their civil war to define Islam.