If you mean that they don't cheat, most of bitcoin casinos have provably fair system, which allows you to check bet outcomes independently (although it become less secure in more complicated games, for example card games.)

On the other hand, there are risks than bitcoin casino will get hacked, seized by police or it's owners may simply vanish with all player deposits and investments. This is the risk with any Bitcoin services (like the most recent Bitfinex hack), so you can only minimize it by storing your money only in sites with the best reputation.