Reputable bitcoin casinos like BitStarz have been online since 2014 and are fully licensed. Your money, whether in your local currency or BTC, is perfectly safe with them.
If you are worried about the fluctuation of BTC relative to other currencies, you can minimize your exposure to cryptocurrencies very easily. By exchanging your bankroll slowly and holding only the BTC you need to play online, you are minimizing your exposure. If you prefer, you can deposit and bet with euros or dollars instead.