To respond to the title of this thread ...
I am on my third Yakima rooftop box, packed and mounted properly on round bars. This is because Yakima's boxes in my experience
consistently do not hold up for long under even mild dirt-road use ... no "four-wheeling," just basic two-tracks and other typical Forest Service, BLM and Park Service unpaved roads that folks herein are familiar with.
This experience concurs with Yakima's position, which is that their boxes are unsuited to use that is anything worse than paved roads, well-maintained and lightly graveled roads, and hard-pack unpaved roads that are in good condition. Use beyond that may void the warranty, according to instructions.
I recently concluded an extended email and phone exchange with Yakima's warranty department in which I asked only for a solution to consistent and continual cracking of their boxes. Unless clamped very securely -- Yakima specifies only "firm" clamping -- I find they will migrate across the bars as well.
They were attentive and responsive, and seemed genuinely interested in the problem. They said the problem is unique to me, even though there are many YouTube videos depicting attempts to patch cracks in Thule and Yakima rooftop cargo boxes. In my case, the cracks typically occur at the four clamping points.
Specifically, Yakima states that their products are unsuited to "off-road use." Since I never travel off roads, and only when necessary do what some would consider rough "four-wheeling," I asked for their definition of the term "off-road" (a term I abandoned decades ago, because it often is incorrectly defined to mean unpaved
roads ... and some people take the term literally).
This was the Yakima's response:
"Anytime you are not on paved or at least well graded hard packed road you are off road." That states the limitations of their products' use.
I sent Yakima the photos below (as well as photos of the cracks). (Sorry for the size!) They depict what for me is normal and frequent use of their rooftop cargo boxes. I don't think it's abuse. So I asked if this fell within their definition of "off-road" use for which their products are unsuited. They said it did.
In the end, I encouraged Yakima to stop posting photos on their website depicting this kind of use (and worse!), and to state clearly in all marketing the limitations of their products' intended use.
I also encouraged them to develop more robust cargo boxes suited to this use, and which still provide anti-theft security. But these were warranty specialists I spoke with, not product managers.
Until they do so, I cannot recommend that anyone spend ~$450 on a Yakima cargo box, and I am actively searching for an equally convenient alternative.