I think with magnets in the roulette, the ball that falls on the number is rigged. Magnets pull the ball to the number that is probably rigged with some kind of computer system.
I am pretty sure that OP was talking about online casinos, as we can rarely see cheats happen in real casinos as they are being regulated and monitored, but at online casinos you can't really know what is behind the code and you can't really know if black/red on the roulette gives you 48% chance to win and not less.
Bitcoin casinos are the most dangerous as most of them are also anonymous and even if you found out that the owner was cheating, you can't really find him and the casino can just disappear without any warnings.