We've seen many cases that the casinos have declined some specific winnings of their huge gamblers for some reasons. They can just say that there's some anomalies or suspected abuse that has happened and we as users can't do anything against that reasoning.
Yes, in this regard, the casino definitely has the upper hand, and it’s unlikely that a player can outmatch it. After all, the casino has leverage over the player—the account can be blocked, or the winnings may not be available for withdrawal and instead only usable for further bets, like a bonus balance.
The player, on the other hand, has no real influence over the casino. What can they do—leave a bad review? Sure, that might be effective when it comes to small casinos that serve a limited number of clients and are highly concerned with retaining each one. But for large-scale casinos with millions of users, such reviews are just a drop in the ocean—nothing they’re going to worry about.