There was some news in the past that certain investors losses their money in gambling sites because it was hack, so it is true that gambling sites can be hack. If big exchanges can be hack and they have more advance security, how much more an online gambling sites?
these days hacking is not just about security because many of the hacks that happened to the exchanges were either inside jobs, than an employee of the exchange did rob them or it was a sham, which means there was no hack ever, and the exchange owner scammed everyone and ran away with their money.
The biggest hacks of all time that happen in both exchanges and casinos are nothing but inside jobs. Remember MtGox, Cryptsy, Mintpal, Bitfinex... list goes on all day. All of those were inside jobs where people inside the system had access to it. Most hacks happen because of that, not because someone finds out an exploit.
