Is there a reason why there is option of depositing 27 different coins / tokens, but only 10 coin types can be withdrawn from the casino?
I can only speculate at this point, as I don't know the exact reason why Rainbet does this. However, I have more detailed insight into another platform, where the reason is as follows:
Deposits are often provided via third-party services like payment gateways (e.g., Changelly, MoonPay), or on-ramp providers that support a broad range of coins, including niche ones.
This makes it easy to accept almost anything. Withdrawals, however, are usually handled directly from the casino's own hot/cold wallets, which are limited to coins they actively manage to minimize fees and of course complexity, because the respective blockchain must also be connected to the platform.
This may well be the reason why you can deposit a large number of coins, but can only withdraw a certain number of coins.
However, the automatic conversion option for withdrawals is actually a great feature that I would like to see on other platforms too!