I've had one of the electric impacts for a couple of years ... it has always loosened any nut i've used it on (including lug nuts on a R3500 Dodge and on a 18' 10K# trailer). I used it last Saturday (not expecting it to do the job) to loosen the axle nut on my Grand Cherokee so I could change a u-joint. The reason I say I didn't expect it to do the job is the torque spec is 275 ft lbs and the book for the impact says it is rated for 230 ft lbs. What the actual torque was on the nut I'll never know now, but I do know it loosened the nut.
Maybe I got lucky and got a good one, or maybe I just have my shop wired right and use the right extension cords so I have full power going to it.