There are probably hundreds of thousands of cameras made each year, in general manufacturing I've heard from various people in industry that they hope to keep defective products under 2%.
With the internet, we who receive one of those 2% make the loudest noise.
Think about it, if those who are really into their cameras, are Canon fans, there is a chance they might be on this forum, if five of maybe 100,000 people get bad copies of a new camera and they were on this forum or another, they would be the first to jump up and down about how Canon model XXX was a failure. Then someone would come on and declare they were going to buy an XXX but since there is such they seem to be a lemon they've decided to go with something else.
What other choice do we have, I for one was held back from getting the 24-70L 2.8 because of this talk, but there really were a lot of people on this forum who said they had heard about those problems. But everyone I know personally has great copies. So when I went to my store they said they had heard of those problems but in the years of selling them they have only had 3 problem 24-70's. I bought one, felt it was back focusing a little and swapped it for another that's fantastic. In truth though, at the same time I sent my camera body in for the same reason and now every thing is good.
I've read so many reports that the 100-400 in notoriously soft at 400 wide open. The only one I've ever used is very sharp-go figure!