Generic drugs can have a bioavailability that differs from the brand name by up to ~10% on either side. According to this article 20% of drugs differ by more than 5%. And, those numbers come from the original studies done to get the generics approved by the FDA, they don't keep testing bioequivalence regularly as part of quality control. This is especially relevant for time-release medications, because you can't just analyze the contents of the pill, you need to test it in humans to get the blood concentration data. There are some special regulations in place for some drugs with a narrow therapeutic index.
I only know about psychiatric drugs - but the FDA has recalled generics of Wellbutrin several times in the past few years for being ineffective (2018, 2016, 2012). Those investigations were initiated by collections of complaints from patients. On top of that, you can find people all over the internet with anecdotes of feeling differences between brand name and generic antidepressants, or between different generic manufacturers. Could be placebo, could be a response to inactive ingredients in the drug (which are not required to be the same), or they are releasing differently or contain different concentrations of the active ingredients.
That's for antidepressants, where you are supposed to notice the effects of the drug. If you can't notice it you could end up taking something for years that causes kidney cancer, apparently. Seems like it would be preferable to prevent this from happening at all rather than letting them sue for damages after the fact. I'm not opposed to generics, I take them, but it's another reason to try to avoid prescription medications in general.