Look through the FDA warning letters database for manufacturers of adderall (f.e. the "Auro" family of producers). You'll notice a lot of warnings have already been issued for quality control and adulteration issues (and continue to be issued yearly, ad infinitum).
Yeah, the 3rd world factory where these drugs are manufactured, for sure tested their product themselves and found it to be bio-equivalent and within the acceptable FDA variance of "how much of the active drug is actually in each pill, vs. what's stated on the specs."
And yeah, the FDA for sure trusts these factories which are thousands of miles away are always up-to-snuff -- even when they don't have an inspector around.
Your statements about the illicit market aren't true either (pro-tip: the legal and illegal market for the usual, non-exotic synthetic drugs comes from the same source; with minor exceptions in local markets).
And a final pro-tip: if you think the only reason brand drugs are expensive is branding/marketing/some other bullshit -- some of the cost is in proper quality control (they're incentivized to maintain high standards, so people keep coming back to ask for the brand-name -- instead of some nameless generic any fly-by-night shop can manufacture).