Being a Duo Owner myself, seeing what's been posted online and on Reddit has definitely concerned me, but doing some research on my own time has led to some pretty concrete and reasonable information.
What is causing the displays to "Crack"?
According to the video attached, it appears that a manufacturing error is leaving one of the rotating pieces chipped and not smooth. As explained in the video, this is causing the hinge to catch and bite, and when forcing the display closed, this causes one part of the hinge to snap, slamming into the back of the display and breaking it. This of course, is a manufacturing issue, not poor design by Apple's part. If we are to believe the figure that 40% of all iPhone Duos made were rejected due to quality control issues, this makes sense, as some would slip through and, if from the same factory, have similar problems.
The device is supposed to be IP68, but X test shows...
From my research on the topic, and obviously I'm not going to use my own Duo for this (I don't have 2 Grand to spare), most people usually refer to Jerryrigeverythings test on "debunking" the IP68 ratings. As shown and discussed by UFDTech, when comparing the test done on the Duo and the Z Fold 8, it appears that the test done on the Duo was not equal and was purposely more severe. In UFDTech's words, the Z Fold 8 received a light dusting of dust/dirt, while the Duo received full caking and large debris specifically in the halfway-open state (Tent position), when the hinge is the most open to the elements and allows for more ingress. As much as I love Jerryrigeverything, it's important to point out the discrepancies.
TLDR - Crackgate, according to the facts, appears to be a manufacturing issue, and the Duo has better dust/dirt protection.
Sidenote - There is a lot more misinformation and purposeful disinformation going on about the device, and while at this point in time I won't go super in-depth about it, I personally try to be unbiased and as factual as possible. If there's something wrong, please don't hesitate to call me out!