With regard to your statement that "That’s a heck of a lot of field use with next to zero online reports of major issues in waterproofness or strength", the problem may be that not all failures are reported.
While I am an enthusiast for cuben, having my brand new cuben pyramid shelter disintegrate under brisk winds and rather hot weather, at the start of a one month camping sojourn in Japan, provided me with direct evidence that failures do occur with cuben, and can be catastrophic.
I emphasize that the manufacturer, although American, was NOT MLD (nor HMG). The tape near the apex started coming off, and in addition, tear marks like a perforated strip near to and parallel to the top of the zipper became evident - possibly where a line had been sewn erroneously, then removed (?). The apex was beginning to disintegrate, and it became necessary to strike the tent and protect my gear, before the failure became complete.
The workmanship in general was pretty shoddy; some seams looked as though they had been sown by a drunkard, in the dark. It made me angry to look at it. The manufacturer, when phoned, tried to assure me that the cuben pyramid should be treated gently, and not be used in too rough a conditions (God forbid!) He refused to refund the purchase price, as the one month's grace period had expired (I'd had heavy monsoon rains and an excessive workload, and so had only been able to test pitch it once or twice prior to starting the trip).
To be fair, he did agree to fix it and return it for free, and I eventually received it with a new door, of slightly different but noticeably yellow color. But in the meantime, I had to purchase a second tent while in Japan, to continue my trip.
This happened 2 or 3 years ago, and I have refrained from making it public, and embarrassing the manufacturer. But it certainly did happen. While I am at it, the bug inner I had ordered with the tent arrived with a hole near the apex that any self-respecting mozzie would have had no problem in flying through, which I had to fix. The tent pole I ordered had been assembled wrongly, so that when folded, it was much longer than it needed to be, which meant I had to remove the elastic, and reassemble it correctly. One end of the pole had been sawn at an angle...
Since then, I have had a much better experience with a Locus Gear cuben Khufu, with excellent workmanship, and with which I am much content; they are currently making me a ripstop 2/3 inner.
I note also that reinforced cuben Khufu and I think Khafra 'mids are being used by alpinists, and in the Himalayas.
I have the highest respect for MLD products and the DuoMid - I regard the LG Khufu and the Duomid as classic designs - but I do think that cuben shelters can cause problems, particularly when there is shoddy workmanship. Just to reiterate, the problems I had were most emphatically NOT with MLD, HMG, or for that matter Locus Gear, all of whom, as far as I am aware, make first rate gear.