As a concept, cashback is a little ridiculous; casinos give back tiny a part of the money that they took in the form of house advantage. Wouldn't it be just easier to have a higher payback in the slot machines?
But then again, it wouldn't look the same
It's funny how some real-money online slots advertise their cashback offers with "Earn money playing slots" to give players a false impression. A truthful advertisement would say "Lose less at slots" or "we've increased our house edge and now you can get some of it back to make it look like we're generous." Then again, that would have a hard time selling.
It could be a good idea to look for them but don't expect a casino with a cashback offer to be automatically better than others.
It's easy for casinos to offer 0.5% cashback on every bet if their payback is 2% lower than others, for example. Or just 0.5% lower, in which case they'd still make the same amount of money as other casinos but would appeal more to certain types of players.
A good rule of thumb would be to be careful about any fantastic offer that you learn about, whether it be free spins, free money or cashback, since casinos with a higher house edge can afford better promotions.
You have a good reason to be especially suspicious when a casino has huge promotions but refuses to publish monthly payout percentages.