Yeah, I agree that it is risky because exchange can change its policies and rules and that may freeze your funds, always be very focused when you work with exchanges.
I do not think that good exchanges will do it, even if a good exchange is going to update or add a new rule or policy then the exchange wont freeze your funds just because the new rules. It can be called as scam if they change rules and freeze user's funds.
Usually, exchange will inform users if they are going to add new thing that may affect user's funds/assets so users are able to do something like withdraw it to their own wallet.