Regarding this "The suspiciously cheap $15 FPGA had equally suspiciously date and lot codes covered (white rectangles on the FPGA chip in the picture above). I have a development board of the same series chip with markings intact, so it definitely shouldn't look like this. It did end up working, but I wonder what the origin of these chips is."

I spoke with a GoPro operations guy once, and asked how you can get the chinese ripoffs at $40, that looked the same as their $400 "real" deal. He said that they literally get chips from scrapyards, like they get batches of memory chips that didn't pass the tests etc. They might work only in a very limited temperature range for example. Also many other components are picked and speced to work 6 months on avg not 6 years.. They know that most of those "gadget" products are bought and used for a weekend and then put in a cupboard anyway.

It would be fine for an R&D project like this but I would be very careful if I was going to make a commercial project (then you also have the politics side, probably fuelled by some paranoia - there are legislations coming up which prevents you from using chinese silicon components in products for certain markets)