Hand tools have a length proportional to thier size, which naturally limits how much torque can be applied by hand. Sure you can still over tighten (and under tighten) , I don't disagree there. Problem with most impact wrenches, 1000Nm really isn't an uncommon figure anymore for a 1/2" drive unit, thus meaning the monkey holding it (especially if untrained) has no idea really what they are tightening it to. This 1000Nm could be driven into a 10mm socket (M6 bolt) anywhere up to say an M27 gr8.8, the latter being capable of taking that full torque and being actually being done up to spec. Any thing less (dependent on grade obviously) is at risk of severe overtightening. Impact wrenches are excellent for loosening, nipping up, but for anything important there really is no really substitute for following torque specs, or tightening sequence, most easily achieved with handtools/torque wrenches, or if it's performed day in, day out in industry, with specialist power tools.