I know that on more than one occasion I have bought an item online that wasn't quite right when I got it. I'm sure, in retrospect at least, that at least some of them were counterfeits. Some of the others were just cheap no-name trash.
Sometimes these products will work just fine and sometimes they won't perform well. Sometimes they can be harmful to other products or dangerous. This is all separate from the moral turpitude of buying knock-off electronics.
Canon, a common victim of counterfeiting, recently sponsored an event at City Tech, the New York City College of Technology, to raise awareness about the problem. They released a survey which showed that US consumers are concerned about counterfeiting, but man, especially millennials, are willing to buy them anyway.
One panelist at the City Tech event was a representative of safety consulting and certification company Underwriters Laboratories (UL), which raises the safety issue: Canon argues that many of the knock-off products are unsafe. The main point has to do with battery packs and external charging devices. This being Canon, the problem is largely about cameras. The pirates faking Canon and other brand-name products also rip off the UL trademark.
The difference in products can be about more than just price. Lithium ion battery packs from real companies with real UL-certification have more than just batteries and cells in them; they also contain a device to sense temperature and circuitry to shut the battery down in case of fire hazard, and the same is true of external battery chargers. Even so, there have been plenty of stories over the years about the potential for Lithium Ion batteries to cause fires owing to bad parts, bad design or other mistakes. A small sample:
These are about the batteries that come from real companies that have reputations and legal interests to protect, and still it's hard to avoid these problems completely when you deal in such large volumes. How much do you think a bunch of anonymous criminals, trying to make every penny they can, will put into quality control?
The survey also showed that most US consumers think they can pick out a counterfeit from the real thing, but in fact it's not that easy. Canon has a "Spot The Fake" quiz on their site. Below is an example; the left is a genuine LP-E6 battery, the right a fake. Can you tell the difference?
The Canon event was about counterfeits, but to me that's not the whole story. I know I've bought no-brand battery packs for notebooks that caused problems; I've no proof, but I believe one of them ruined a Thinkpad motherboard. That's the last time I buy a battery that doesn't at least purport to be from a reputable manufacturer.
This same problem has been true of printer ink and toner for some time. Both knock-offs and compatibles are widely available, as are refilled cartridges and kits for refilling. There's not a safety issue with them, but there definitely may be a problem with quality. It seems there are also counterfeit printer ink/toner devices, so that's morally problematic.
Now it's also obviously true that Canon and even UL want you to buy genuine products in part because, unlike with knockoffs and compatibles, they make money off the genuine hardware. This is something to bear in mind, but not a reason to discount the safety or moral arguments.
I find it very easy to believe that the smaller merchants I've never heard of are more likely to sell knock-offs and products which don't necessarily meet the standards I'm expecting. Based on this, my own experience and even materials from Canon and UL, I'm now less likely to buy the cheapest option or from someone I've never heard of.