I don’t recall ever getting one denied. Maybe I was lucky but those CD rebates were huge for me as a kid to have infinite supply of media to write content to. Music, games, apps, etc.
I still have several spindles of CD-Rs (and later DVD-Rs) that were free (or maybe $1) after rebate.
Sometimes they insist in paying in amazon vouchers or some other 'almost cash' currency.
They are very common on subscriptions.
Other things used as cash include gift cards, cigarettes, drugs, bikes, iDevices, baby formula, diapers, laundry detergent...
Although, generally speaking, Amazon and other general purpose large retailers (Walmart, Home Depot) have figured that out. When I get rewards points of some sort with an option to convert to various gift cards, the best deal is always with the specialty retailers but then you need to make a point of using the card for something you really want/need as opposed to just adding some dollars to your Amazon account.
I usually find out about these deals on https://www.doctorofcredit.com/ which is run by what seems like fantastic, non-scammy, people and its the only credit card blog that I know of that's independent. The rest shill for referrals.