1st rule of crypto is control your own keys - if you are using an exchange as a wallet - you will loose money. It's been proven over and over again. Unlike dollars, you are 100% responsible for your coins. Invest and use at your own risk.
I've had a very positive experience with polonex. Do I ever store coins there? Only for the 10 minutes I need to make the exchange, then directly moved to my private wallet. They never hold more then I'm willing to piss away.
Take care out there!