Which is why, to a great extent, they seem to rely upon IEDs and rockets on timers, as opposed to direct physical assaults. Of course, they do conduct direct assaults, but they have to be very careful about not concentrating too early or dispersng too late, or going head-to-head against Coalition firepower. I think that you are generally right about the lack of cover afforded by the terrain, but the point is that in order to gain a significant political or military effect, they have to strike where people will quickly become aware of the results (and the collateral damage risk to the Coalition is higher), so they need to operate near areas of significant human presence: cities, towns and villages.
Cheers