Probably over a million people have taken curcumin+piperine supplements that they bought via Amazon.

See, e.g.: https://i.ibb.co/Qvbdjqf7/amazon-curcumin.png

My impression as an author with books on Amazon is that you get one review or rating for every 20-30 purchases, and there are two products with "200k people have purchased multiple times."

And that's to say nothing of people who have bought such products via other sources, like Wal-Mart, etc.

All that, and you have "several" cases? That's not a lot. You need much stronger evidence to support any assertion that the nature of the product is harmful. (As opposed to contamination or chance effects.) If anything, I'd say that purchase and utilization statistics support the notion that the product is not harmful.