Things like this are pretty much the reason I am sticking to branded products, despite their increased cost I'd prefer the peace of mind.
I recall recently that some website was selling some kind of minoxidil formulation, but some company (I think the American version of "trading standards"?) found that the formula was far from what was advertised.
At least with branded products you know they are tried and tested and will apply strict quality control measures.
Here's the thing...it's worth it for someone to make fake Adderall. There's a secondary market for that stuff, and the pills themselves are worth MUCH more than a pill of finasteride.
You can't go and sell finasteride on the streets to college kids who need to stay up all night studying. Basically, it's so cheap to make generic finasteride tablets that they would basically lose money if they did so. The risk isn't worth the reward.