Instant Deposit is not secure as long as someone can perform a double spending attack
I have already seen some sites that have changed from instant to one confirmation because of people trying to scam
People attacking sites that requires 0 deposit will get nothing from that double spend attacks as they are not able to withdraw it. Its just a free play for them
People can abused this. If they win then they will push through the transaction by sending in an input but if they dont than they wont send in an input. Repeat this process if you lose by making new account, Keep on repeating this and it will make things easier since you dont need to wait for it