Those are just a guide lines what is happening in blockchain and while choosing fee you also should look at how many unconfirmed transactions in the blockchain. If you see a lot of unconfirmed transaction means you shouldn't follow what they suggested instead pay a slightly higher fee than what they suggested to get your transaction confirmation faster.
I do agree that there will be some delays but 24 to 48 hours is too much that shows the fee paid for those transactions was way below than the expected fee.
I'm sure that you didn't actually take a look at the 21.co site because its recommended fee is always the lowest one marked in green (the 0-1 estimated block fees).
Let's also note that for casinos to deal with multiple inputs, it jacks up the size of the transaction to ridiculously high levels if a user is withdrawing.
Here's an example of a withdrawal that I requested from Stake: 2f1459fcc40cb6af51a5382a1f4847a1bbf7d8390dd4ffd3ef82b0ab1379ec2d
This transaction has a size of 1847 bytes which is ridiculously high. Even with the 2.781 mBTC fee that they used it's still 150 sat/byte which is a low fee. If you wanted to get the transaction as quickly as possible (according to bitcoinfees) it would require a fee of 0.00777587 which I don't think anyone is comfortable with unless the withdrawal is >8 BTC.
Agreed, transaction confirmation is very low even with high priority fee (high priority = high fee = quick confirmation time). Bitsler pays normal fee, for me that's good but there are many people who don't likes it. I think withdraw fee system needs to be changed, transactions are really huge bytes as actmyname mentioned. I think bitsler has to give users ability to choose satoshi/byte fee. After that there won't be dissatisfied players.