This is true, there are some casinos out there which when one of their gamblers obtains a big win they try to find even the smallest detail they can to try to refuse paying the money they owe, and one way to do this is by asking way more than what is needed for a standard identity verification, with the hope the gambler fails to provide the needed documents and then they save themselves a lot of money while still protecting their reputation, as they can later claim it was the gambler the one that failed to pass all their tests and they were justified on their refusal to pay them.
In such casinos, players simply will not play. All players play at the casino only in the hope of a big win, but if they know that they will not be able to withdraw their winnings anyway, they will not play at this casino. They will go to another casino.
This is also true, but then, the problem now is, how does the player know beforehand, the casino that wouldn't allow them withdraw their big winning?, this is where looking out for reviews become very important before deciding which casino to play on, but then also, good reviews is not a 100 percent guarantee that a casino would perform such a trick on a gambler, considering the fact that requesting customers to pass kyc verification is something casinos do on an individual level, they can easily decide who to carry out a kyc fraud on, and who not to, depending on how large a customer's winning is.
It takes a casino with much integrity to never do such to any of their customers, no matter how large the customer's winning is.