My speculative theory is that some recently produced R7800s have a bad batch of these Flash chips.
Yes, if the Macronix NAND flash chip is faulty/less robust/weak, etc., I suspect it is only a batch of them--not all of them. Indeed, since this occurs ACROSS product models (just inspect the 35+ threads on the Netgear forums for proof of this...), it would seem to point to a defective component that is shared between these product models, not just the R7800, i.e. a bad batch of flash chips (or perhaps another component).
We could figure this out at this level if we start to pull together data. For example, if we could document:
1.) The approximate purchase date of the router (if new, and perhaps an estimate of when it was from if not);
2.) The actual production date of a router according to "artmtd -r protime";
3.) Potentially the serial number (minus the last four or five identifying digits) to see if we can limit the issue to a particular range;
4.) The manufacturer ID of the NAND flash chip either obtainable through
@kamoj's beta add-on, or via early boot dmesg capture;
5.) Potentially other ancillary information about how/when the problem first appeared like out-of-the-blue, after a flash process, during heavy usage, following a power outage or brownout, etc.
For sure if we have this info, we could really start to narrow in on the problem. I also recognize that not all information would be available about the routers that are affected--some have already been returned for RMAs, some have probably been tossed or discarded, some details might not be so easy for folks to obtain, etc. But if we could fill in some of these details, I think the problem would come into focus, and for sure, we could start to eliminate some of the possibilities that have been discussed.
Not that I have time or desire to do this, but I recognize that actually getting this done requires time and energy. I would be willing to collect and collate information, and put it in a public place for everyone's inspection. Indeed, I will make a first start this evening, and contact those who have already provided information to make sure that they are okay with this before I actually include anything they might have contributed either publicly or via PMs.