A lot of smart contracts are compiled WASM, unless I have the source code and recompile for myself then check bit for bit I really can't make sure it's fair.
But developers don't really want to release the source code, none of the EOS gamble sites do, you'll have to trust them to play.
This boils down to the question: is smart contract really necessary?
You can develop a centralized application that users can't check fairness but can use crypto as a payment method
This will be no difference to a decentralized application that users can't check fairness. To use the dapp, users will have to visit your site to interface, which is centralized.
Don't forget your transaction history will be used for data mining if you use dapp, don't expect your identity wouldn't get correlated to your blockchain activity, it will, because exchanges have stricter KYC than banks.
But developers don't really want to release the source code, none of the EOS gamble sites do, you'll have to trust them to play.
This boils down to the question: is smart contract really necessary?
You can develop a centralized application that users can't check fairness but can use crypto as a payment method
This will be no difference to a decentralized application that users can't check fairness. To use the dapp, users will have to visit your site to interface, which is centralized.
Don't forget your transaction history will be used for data mining if you use dapp, don't expect your identity wouldn't get correlated to your blockchain activity, it will, because exchanges have stricter KYC than banks.