Most of the people that are earning good amount with casino's are the ones that invested bigger amount, estimating higher than 1 BTC is the recommended amount of invested. And it can grow 5% for the 6 months and chances of not growing is still there. It depends on how big the bankroll is and how many investors are there too.
It also depends if the casino has a really good player base and there are a lot of high rollers. I tried investing in bitvest, they have a high roller there and I caught him on a bad streak. So my money almost doubled as I set my investment to the max risk they have which got me x5 of what my investment is.
Also, if you invest in casino's that have lower-risk bankroll management (limiting the max win-rate on bets), you won't have high rollers taking out your investment.
Crypto-Games does this btw. Chances of losing your investment are really really low.
@michkima every single investor in bitvest is on 5X, i think it is technically 1X