i will not surprised if right now when the bitcoin price skyrocketed there will be a lot of hacking attempts, always becareful!
I believe not all funds are kept in their hot wallet
At least half should be kept in a cold wallet, that is basically hack-proof
Also, websites containing millions of $ in bitcoins probably thought about security and invested significant funds into security
Which makes them hard to hack, ofcourse it can be done, it requires a lot of skills you won't be able to obtain in a few years time
You'd also need to be able to hide your traces if you don't want to end up in jail
Nothing is unhackable, I guess. But there is really not much point in hacking gambling sites as most of their hot wallets contain pretty low amount and they surely don't store their cold wallets in their servers. I also agree that the websites have invested a lot of money to prevent this kinds of hacks, so it would be easy but probably doable.