The two knockoff Neopixel test fixtures went dark while their USB charger accompanied me on a trip, so they spent a few days at ambient basement conditions. When I plugged them back into the charger, pretty much the entire array lit up in pinball panic mode:
Turns out three more WS2812 chips failed in quick succession. I’ve hotwired around the deaders (output disconnected, next chip input in parallel) and, as with the other zombies, they sometimes work and sometimes flicker. That’s five failures in 28 LEDs over four months, a bit under 3000 operating hours.
For lack of a better explanation: the cool chips pulled relatively moist air through their failed silicone encapsulation, quietly rotted out in the dark, then failed when reheated. After they spend enough time flailing around, the more-or-less normal operating temperatures drives out the moisture and they (sometimes) resume working.
Remember, all of them passed the Josh Sharpie Test, so you can’t identify weak ones ahead of time.
The Smell of Molten Projects in the Morning 
Plastic optocouplers (and, I seem to recall, IR comm chips) use silicone over the chips, then would encapsulate the assembly in an epoxy. Pretty sure that played out for the IR chips, but definitely the case for plastic ‘couplers. Those would pass life tests: high temp burn in, and the dreaded 85 C/85% RH test. No idea if anybody encapsulates neopixels in epoxy, but that way would work, at a substantial cost premium.