For the gambler provably fair only means you have a tool to verify your bets. If they were skipping nonces, changing server seeds, etc you would still be cheated. Only if you check all your bets, you can find out that you were (or not) cheated.

For the investor there is no way at all to verify anything. In theory it is very easy for the owner to fake a winning whale with the server seeds (or just small winners .. every day.) I actually started a discussion about that https://bitcointalk.org/index.php?topic=774458 "Provably fair for investors?", but the only solutions so far are not really provably fair, just slightly less risk.