I think, you might be needing a lot of funds when investing on gambling site because they have minimum funds to be accepted and its kind a risky move because its all or nothing and win or lose situation. However , gambling is not an investment but a game where money counts and not just a number.
The minimum amount required to invest in gambling site's bankroll is 0.01 BTC(some require even less) and it's not that risky, over long-run you will make profit. the more you'll invest the bigger returns you're going to get.
It is still risky but yes I agree that the more you invest regularly, you will have more chances of mitigating the risk and potential earning than just investing one time and forget.
The problem with investing just one time is that, there might be some lucky shark that will win big and you lose on that however, if you keep investing, you will get even and even earning more in the next run.