A poor quality wrench will strip the head off of a bolt much easier than a high quality wrench from Snap On.
Years ago when I was working as a car mechanic, the Snap-on dealer used to give a very convincing demonstration of this - he had a hex bar of hard nylon and would put one of his ring spanners on it and one of any other competitors, and then twist one against the other. The non Snap-on was always the one to round off the bar.