On top of what @str11ngfello posted I would like to add that you should always be worried about exchange. Any coins that you don't have yourself are at risk. You never know if there will be any hacks or bankruptcy.
That being said a lot of people don't feel comfortable or aren't confident about storing the coins by themselves. It's kind of an analysis that you need to do for yourself. I have some coins on exchanges but most of my Bitcoin for example I keep in a Trezor (hardware wallet).