The units can be programmed using the ST-LINK/V2 interface, so they aren't complete duds.

I fixed one, but still Arduino programming over USB doesn't work (regardless of pull-up resistor value), can't tell, if other USB functionality works. Said that, the batch of 5 (ordered, 6 shipped) I received had indeed a 10kOhm USB pull-up resistor, when you find all over the 'net that it ought to be 1.5kOhm. Probably just a typo, but perhaps indication of sloppy spread sheet reading. afaik, there is no STM32Cxxx and modules with the STM32F103C8 are referred to as Blue Bill. Many of us are aware of the STM32C103 boards referred to as the 'Blue Pill'. It isn't integrated into a single die with the ARM core + peripherals like a STM32. The GigaDevices GD32 clone of the STM32 has a separate flash die inside the chip they can swap out easily. The price difference is tiny and just not worth the potential headache if that additional flash is actually marginal in some way. Always buy the proper part for production use. I wouldn't use the additional 64KB for production though. Presumably ST made a new revision at some point with a die shrink or redesign which is why some 64KB parts do only contain 64KB. The additional 64KB may be untested or it may be perfect but ST aren't going to guarantee it even if it perfect. In this case, ST would have been making STM32F103CBs which has 128KB flash and packaging them as STM32F103C8s which officially only have 64KB flash. It is a balance between making multiple designs with varying amounts of flash and making more of the larger part and sell it as the lesser part because it is cheaper or yields are better.

Stm32 blue pill schematic