Here's the deal....

Some lenses, but not all, will have focusing problems with some cameras, but not all.

It's not a black and white thing. The lens and camera are great. If it turns out you think you have a problem you can send them both to Canon for repair/adjustment. I've had to do it myself in the past.

Yes, it's a valid concern and it's REAL. The extremes of most arguments come across on the internet which in this case the two extremes are "All canon cameras or lenses have focusing problems" and "the back focus is the newbie photographers fault". Usually, the truth lies in between the extremes.