Centralized exchanges have never been secure, not before, not now. The reason is very simple, if you store your coins on the CEC, then these coins are not actually yours, you just can see the balance that the exchange displays for you, but in fact you have no guarantees that you will be able to withdraw your coins at any time when you will need it. In addition, you may be required to pass verification, which is also not very good.
Too simple yet too hard to swallow.
If you are a trader, then maybe this doesn't affect you at all but if you are a mid to long term investor, putting and storing it on a centralized exchange isn't the best option for you. I'm not a trader at all (although I tried to be one, but I failed) that's why I'm not putting any coins into any centralized exchanges.
Just think about this OP, once you put your coins into any centralized exchange already, it isn't yours already therefore it isn't safe. Not your keys, not your coins. Just use exchanges whenever you will buy or sell your coins, but never store it there for a long time.