The argument there is that while that is true, the inclusion of the lock includes more parts and thus more chances for the gun to fail at a critical time.
Most reports of the lock failing are of the "a friend of a friend had one!". Local gunsmith told me that all of the guns brought in to him because of "internal lock failure" were locked up because of something else.
I'm not saying it has never happened or doesn't happened, just that it doesn't happen as much as few want us to believe. The argument against the lock has kinda drifted from the possibility of failure to one of "esthetics" or "principle", you know, the "ugly Hillary Hole". When the lock first came out, it was to be the doom of S&W unless they immediately succumbed to the pressure from complete lack of sales. Ruger's stock dropped ten times the rate of S&W stock today. Just sayin'......