Nothing is really bad about China or imported glass. What makes someone say this? Well, the idea that jobs are sent over seas. But this also allows people to get into the glass game at a more affordable price.
Another thing China glass or import glass does allow is that blowers can use parts, color or things to help bring their costs down, lowering the cost to the end buyer.
China glass my also be called bad for the reason that many shops and things like DH gate sell lesser quality glass. It may seem like a deal, but you get what you pay for. Glass many times can become a second quality item or worse. This is where the bad China comes from.