Similar to the poor quality shells on the market, the protective glass (or even plastic) lenses on the market as just as bad.
Some issues include poor fitting with original shells, untreated glass (leading to easily breaking or scratching), poor quality mix leading to more opaque glass (less clear) and low quality prints.
We had to have better, so we found a glass factory willing to work with us, and we now make our own glass for all consoles. Our glass is heat treated so much stronger than normal glass, more scratch resistant than normal glass, high quality image prints, high accuracy finishes and very clear optical properties.
The difference a high quality, well fit glass lens makes to your final product is huge.