I don't know how to say this, but for example, you think that your coins are dirty, but when you exchange them, they turn out to be clean.
This often happens, but you shouldn't hope that it will always be like this. Sometimes you need to test theory in practice.
It goes the other way as well. You think there is nothing even slightly suspicious about these coins I have and then they can get frozen and the exchange starts asking questions. I am speaking in general terms and not pointing fingers at CCE.Cash here. Not everyone will be interested in testing and playing with their money just to see if the service will allow them to swap or not. It's risky, especially if you don't want to reveal anything about yourself to the exchange or can't explain the origin of the coins.
If I had a shop and my business policy was that I can allow you to shop but maybe I will confiscate your money and call the police on you for some reason, you would never walk into my shop. Somehow this kind of behavior has become acceptable in crypto.