Yeah, all of the aforementioned cards of mine were in mining rigs, and honestly, I just haven't had the energy to pursue the issue. I didn't want to say anything pre-maturely and start rumors until I had a chance to dig out each of the cards and the mobos, and do some A/B component testing to find out first exactly *what* died, to cause the rig(s) not to turn on.
For example, the rig with the two RX 6600 cards, was in a rosewill case with a case PSU with a dual molex to 8-pin being used to power the second card. It's possible that the PSU just doesn't have enough gumption anymore to power up the two cards.
So, I HAVE NOT done proper "process of elimination" to determine cause of death of these components. They were in rigs that had been run hard for over a year, some nearly two. Failures can happen eventually.
I did find it weird though, that two of my rigs with RX 5700 cards were no-power-on anymore, and died within a day of each other, AFTER updating drivers, when they had been basically FINE for over a year. It did make me suspicious of the most recent drivers being card-killers, but I didn't want to start un-substantiated rumors.
Edit: That being said, there really hasn't been any examples that I know of within the mining realm mentioning RX 6600 cards dying en-mass, though I suppose that the number of rigs running Win10 and Nicehash at this point and still being run to be exceedingly small; most serious miners use HiveOS. It's quite possible that my dodgy old mining rig "shells" and worn-out and cheap PSUs aren't doing my cards many favors. I wouldn't be all that surprised if my card failures weren't due to either a Nicehash update (they control power limits and fans, in some cases), or just my dodgy builds.
Though, the fact that they all happened around that same time, and also, after I decided to go around and update the driver on all of those rigs, because the newer Nicehash version wanted newer video drivers, well, I don't know.