I have a 4x2 stack and a 3x2 stack and 4 of his older style single 4x2 cages stacked.
Of the old style, I got some sagging/warping but that was because I placed a very heavy piece of wood on top the stack for years (oops).
Of the new style, which are these cage stacks, overall I like them. I have not had any sagging, warping, leaking or any kind of issue like that. Been using them 8-10 years now?
He did change the type of plastic used and this new one is kind of porous. I prefer the old plastic. I used a bad choice of caulking so with that and this type of plastic, I do have some staining issues. That is my main complaint.
I may also drill some extra holes for added airflow. Otherwise, I think they are fine.