I have owned the device for almost a year and there are no cracks, not even scratches. I have to say that I'm not really very careful with the device. It can withstand a lot.
So it doesn't seem to be a construction fault.

Well, I've had the device since launch and have never had any cracks, nor have my colleagues who also bought the Z3.

I've had the Z3C since December 2014 and it has fallen on the floor once. Nevertheless, I don't have any cracks.

The back of my device has also shattered twice.

Unfortunately, my display broke despite the protective cover and display film. This was caused by placing the phone on the table. But otherwise a top device.

I've had the phone since April 2015 and it's not cracked or damaged in any way. It's a "pocket device", so it's always exposed to stress when sitting in my trousers. The glass has not been scratched even though coins and keys are in the pocket - top quality from the point of view.

The back is very sensitive on mine and has already been broken and replaced 4 times. I must say, however, that I do not know whether the replacement glass is really as strong as the original glass.