As soon as possible then, you should be moving your coins to a wallet which you control. Otherwise any problem with their fee estimation system is a problem for you. If you don't own the private keys, you don't own the coins.
Is there a risk to leave my coins and Alt-Coinson the trading platforms like Gdax, HJitBtc and Bittrex?
What is the risk ?
Isn't it better in case of hard fork from one of the currencies anyway?
Chris