So the Taliban need only to build a trebuchet and throw stones at the machine until the mirror breaks? They ARE used to stoning things after all.
But seriously, an inert projectile isn't going to care much how hot ite gets. In fact a molten projectile might be more effective than a cold one. Flechette artillery rounds would, I suspect, prove very hard to disrupt. Indirect heavy machine gun fire would keep the firer out of LOS of the laser while doing significant damage.
The usual issues over target detection, identification and damage assesment remain. Gonna be a lot of toasted avians in the vicinity of these devices. And if a bird filter is included, hows about some balsa wood explosive.gliders? Ground clutter and terrain masking will add to the problems of detecting and tracking a low level target.
Regardless of the laser wavelength, it is probable that a suiable obscurant can be found to significantly degrade the effectiveness of the beam. (Apart from natural problems of rain, fog, cloud or dust.)
Unless this has almost infinitely fast beam pointing, multiple units will be needed in any given location. One suspects that they aren't cheap. As the Strategic Defense Initiative has shown, the cost of defensive measures may exceed the cost of saturation attacks.
Taliban in tinfoil suits will. of course be able to attack with impunity. (And be immune to mind-control satellite feeds.)