I am very disappointed with my Makita 1/4" impact driver. I'd say 8 out of 10 times that I use it, it ends up breaking the head off the bolt / screw before it even gets tight. Just this past weekend I tried driving in some 3/8" lag bolts. I still had about 1/2" of lag bolt to go and "pop" the bolt head breaks off. It seems like the ultra-fast pulses of the impact fractures bolts.
Meanwhile, I've used my 1/2" air impact for similar duties and rarely have any trouble with fractured bolts.