I'm not sure I buy all of those arguments. Power supplies (cheap ones, anyway) are, indeed, of pretty mixed quality. But their failure rates show (in my experience, anyway) a distinct "bathtub curve", essentially failing within the first few months or not for several years. For a part like that, a used one can actually be more reliable than a new one, because you're essentially getting the "burn-in" for free.

And while, again, it's true that off-brand memory has higher error rates than premium stuff (which, generally, is coming out of the same fabs but testing at higher tolerances), the error rate is (1) still very low, I've only seen one bad DIMM over the last 10 years or so, and (2) trivially easy to test with memtest86. Got a bad stick? Yeah, it sucks, but you just replace it (for $20 or whatever) and move on. It's not a reason to triple your memory cost, IMHO.

And disk drives, in my experience, simply don't have a premium/bargain distinction in quality at all. Instead, these things fail in batches. Some 4500 RPM clearance models will churn for years, while some models of 15k RPM SAS monsters like give up the ghost within a few months. You can't win with drives.