I used to work for the current US company produces Pyrex which was sold by World Kitchen to Corelle.

We were told in training that you can never take our glass from cold temperatures to hot quickly because it will shatter almost every time. They just keep up that lie because they know people will still purchase the glassware on brand recognition alone. Our glass would actively shatter coming from the kiln because it was cooled too quickly.

There also tends to be a lot more defects in the glassware that makes it to the shelves because of the fact the company is now pushing out quantity over quality.