As all Lightning network transactions are settled on the Bitcoin blockchain they inherit the same level of security as Bitcoin itself. Arguably, Bitcoin is the most secure monetary system ever invented and therefore any Lightning channels which are settled on the Bitcoin blockchain are highly secure. That said, the transfer of value on the Lightning network itself is largely untested at this stage, and there may be issues which are found further down the line. The Lightning network is currently undergoing heavy testing from the Bitcoin community, but it is safe to say that once it reaches production it will be secure for Bitcoin casino deposits and withdrawals.