Basically, the term "not your keys Not your coins" works more on issues like this, now you have to know that you only have passwords or whatsoever that they may offer you as key but true control is in there hands o you have to be aware that it's not safe no matter the kind of exchange, Decentralised open source exchange or centralised closed source none is safe for a long term holding! it's only safe for a short period of time especially when you do business that requires in and out so you can't use your cold wallet for all that instead you can use hot wallet and exchanges since it's an immediate use.