15% apy is amazing, but not out of risk.
Although Blackjack.fun is a consolidate casino operating since 2018, there is always a risk of hacks, legal problems or even financial problems.
Yeah, this offer is good for people who play in the casino, sometimes you could have funds you are not ready to gamble with yet, or maybe the network fee is high, then it is a good idea to stake your coins in the platform, until you are ready to move it out. But it may not be necessary or safe to move your coins out of your self custodial wallet into a casino, just for staking
only, if it is to gamble, then that is fine, because there is always risk when you are not in control of the keys to your funds.