One of the big things I don't see mentioned here is that while the "active ingredient" in the medication is the same, all the other stuff in it is not. This also includes the binding agents that hold the med together and also the release mechanisms that control how/when the medicine is absorbed into your body.
For example, generic extended release amphetamine salts don't release into your body in the same way Adderall XR does, despite being the same medication. While the patent on the drug has expired, the patent on the proprietary release mechanism has not expired. So generics can't copy it. This could cause the generic to not work as well for some people, or some people could have a reaction to those different ingredients.
I was going to say, it does depend on the drug and person. My son had that experience where the insurance flip-flopped to cover generic instead of Adderall, but it did not work at all for him so we had to fight to get it changed back. Since then every year or so insurance plays their game and we have to go through the ritual explaining why it can't be generic when that becomes the one covered. It shouldn't be this hard, right?
Omg. We fight the damn battle every year as well for our son. They don't understand that the delivery systems are different. Well, they probably do, but don't care.