trading platforms are safe against hacks and other forms of attacks but that doesn't mean they are 100% safe. nothing is 100% safe even big companies with specialty in security get hacked.
what is not safe regarding trading platforms is the owners themselves. they can scam you at any moment and run away!
More secure is to save all your assets from encrypted currencies in local or hardware,
this has nothing to do with your title. a trading platform, as the name suggests is a platform for trading not for storing if someone is storing their coins there then they have not understood what is what in crypto world.