A pessimistic analysis from Jane's, on both Sunni and Shia opponents of Daesh:http://www.janes.com/article/45284/s...-islamic-state

Here is one passage:
...neither the Peshmerga nor Shia militias have either the means or legitimacy to assert authority over the substantial swaths of predominantly Sunni territory that the Islamic State currently controls in conjunction with local Sunni insurgents.

In short, the current array of Sunni groups and forces opposed to the Islamic State across Iraq is too weak and too localised, and is lacking in the wider credibility required to constitute an effective fighting force.