That is not the biggest issue about holding your bitcoins at exchanges, if the website is programmed corretly, and secured really well, then the chance of hackers break-in is kind of small.
Most of the people are against storing the BTC at exchange wallet, because you don't have access to the private key of the specified address.
It means that in the scenario when the website stops working, you won't be able to retrieve your money by yourself, only support of the exchange can help you (if it is on, while site is suspended).
Thats the most important reason why people avoid using crypto exchange wallet.

If you are in possession of these keys, you are able to get your funds in every place, just by exporting the privatekey to the wallet.