Just search for trading exchanges being hacked in the past and make your own conclusion. Of course its not safe to put all your funds not just in Coinbase but in any exchanges because if the hackers were able to get in, I'm sure that all you money will be lost forever.
Besides they can even lock your account without an appeal or in worst case scenario, authorities seize the servers and the hard drive of a exchange for suspicious activities, resulting in your funds being held for a long time. If you are holding a lot of money, its not really recommended to put it in a exchange, but in a hardware wallet where you can have total control.